lang, engine: Add a facility for resources to export constants

Since we focus on safety, it would be nice to reduce the chance of any
runtime errors if we made a typo for a resource parameter. With this
patch, each resource can export constants into the global namespace so
that typos would cause a compile error.

Of course in the future if we had a more advanced type system, then we
could support precise types for each individual resource param, but in
an attempt to keep things simple, we'll leave that for another day. It
would add complexity too if we ever wanted to store a parameter
externally.

Lastly, we might consider adding "special case" parsing so that directly
specified fields would parse intelligently. For example, we could allow:

	file "/tmp/hello" {
		state => exists,	# magic sugar!
	}

This isn't supported for now, but if it works after all the other parser
changes have been made, it might be something to consider.

README.md

@@ -21,7 +21,7 @@ ensure that your file server is set to read-only when it's friday.
 import "datetime"
 $is_friday = datetime.weekday(datetime.now()) == "friday"
 file "/srv/files/" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	mode => if $is_friday { # this updates the mode, the instant it changes!
 		"0550"
 	} else {

docs/faq.md

@@ -242,7 +242,7 @@ gets created in case it is not present, then you must also specify the state:
 
 ```
 file "/tmp/foo" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => "hello world\n",
 }
 ```

docs/language-guide.md

@@ -206,7 +206,7 @@ value to use if that boolean is true. You can do this with the resource-specific
 $b = true # change me to false and then try editing the file manually
 file "/tmp/mgmt-elvis" {
 	content => $b ?: "hello world\n",
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 ```
 

engine/resources/file.go

@@ -35,16 +35,46 @@ import (
 	"github.com/purpleidea/mgmt/engine"
 	"github.com/purpleidea/mgmt/engine/traits"
 	engineUtil "github.com/purpleidea/mgmt/engine/util"
+	"github.com/purpleidea/mgmt/lang/funcs/vars"
+	"github.com/purpleidea/mgmt/lang/interfaces"
+	"github.com/purpleidea/mgmt/lang/types"
 	"github.com/purpleidea/mgmt/recwatch"
 	"github.com/purpleidea/mgmt/util"
 	"github.com/purpleidea/mgmt/util/errwrap"
 )
 
 func init() {
-	engine.RegisterResource("file", func() engine.Res { return &FileRes{} })
+	engine.RegisterResource(KindFile, func() engine.Res { return &FileRes{} })
+
+	// const.res.file.state.exists = "exists"
+	// const.res.file.state.absent = "absent"
+	vars.RegisterResourceParams(KindFile, map[string]map[string]func() interfaces.Var{
+		ParamFileState: {
+			FileStateExists: func() interfaces.Var {
+				return &types.StrValue{
+					V: FileStateExists,
+				}
+			},
+			FileStateAbsent: func() interfaces.Var {
+				return &types.StrValue{
+					V: FileStateAbsent,
+				}
+			},
+			// TODO: consider removing this field entirely
+			"undefined": func() interfaces.Var {
+				return &types.StrValue{
+					V: FileStateUndefined, // empty string
+				}
+			},
+		},
+	})
 }
 
 const (
+	// KindFile is the kind string used to identify this resource.
+	KindFile = "file"
+	// ParamFileState is the name of the state field parameter.
+	ParamFileState = "state"
 	// FileStateExists is the string that represents that the file should be
 	// present.
 	FileStateExists = "exists"
@@ -975,7 +1005,7 @@ func (obj *FileRes) sourceCheckApply(apply bool) (bool, error) {
 				// programming error
 				return false, fmt.Errorf("not a Res")
 			}
-			if res.Kind() != "file" {
+			if res.Kind() != KindFile {
 				continue // only interested in files
 			}
 			if res.Name() == obj.Name() {
@@ -1582,7 +1612,7 @@ func (obj *FileRes) Reversed() (engine.ReversibleRes, error) {
 func (obj *FileRes) GraphQueryAllowed(opts ...engine.GraphQueryableOption) error {
 	options := &engine.GraphQueryableOptions{} // default options
 	options.Apply(opts...)                     // apply the options
-	if options.Kind != "file" {
+	if options.Kind != KindFile {
 		return fmt.Errorf("only other files can access my information")
 	}
 	return nil

engine/resources/resources_test.go

@@ -228,7 +228,7 @@ func TestResources1(t *testing.T) {
 		p := "/tmp/whatever"
 		s := "hello, world\n"
 		res.Path = p
-		res.State = "exists"
+		res.State = FileStateExists
 		contents := s
 		res.Content = &contents
 
@@ -292,7 +292,7 @@ func TestResources1(t *testing.T) {
 		res := r.(*FileRes) // if this panics, the test will panic
 		p := "/tmp/emptyfile"
 		res.Path = p
-		res.State = "exists"
+		res.State = FileStateExists
 
 		timeline := []Step{
 			NewStartupStep(1000 * 60),      // startup
@@ -316,7 +316,7 @@ func TestResources1(t *testing.T) {
 		res := r.(*FileRes) // if this panics, the test will panic
 		p := "/tmp/existingfile"
 		res.Path = p
-		res.State = "exists"
+		res.State = FileStateExists
 		content := "some existing text\n"
 
 		timeline := []Step{
@@ -811,14 +811,14 @@ func TestResources2(t *testing.T) {
 	testCases := []test{}
 	{
 		//file "/tmp/somefile" {
-		//	state => "exists",
+		//	state => $const.res.file.state.exists,
 		//	content => "some new text\n",
 		//}
 		r1 := makeRes("file", "r1")
 		res := r1.(*FileRes) // if this panics, the test will panic
 		p := "/tmp/somefile"
 		res.Path = p
-		res.State = "exists"
+		res.State = FileStateExists
 		content := "some new text\n"
 		res.Content = &content
 
@@ -850,7 +850,7 @@ func TestResources2(t *testing.T) {
 		res := r1.(*FileRes) // if this panics, the test will panic
 		p := "/tmp/somefile"
 		res.Path = p
-		//res.State = "exists" // not specified!
+		//res.State = FileStateExists // not specified!
 		content := "some new text\n"
 		res.Content = &content
 
@@ -883,7 +883,7 @@ func TestResources2(t *testing.T) {
 		res := r1.(*FileRes) // if this panics, the test will panic
 		p := "/tmp/somefile"
 		res.Path = p
-		//res.State = "exists" // not specified!
+		//res.State = FileStateExists // not specified!
 		content := "some new text\n"
 		res.Content = &content
 
@@ -907,14 +907,14 @@ func TestResources2(t *testing.T) {
 	}
 	{
 		//file "/tmp/somefile" {
-		//	state => "absent",
+		//	state => $const.res.file.state.absent,
 		//}
 		// and no existing file exists!
 		r1 := makeRes("file", "r1")
 		res := r1.(*FileRes) // if this panics, the test will panic
 		p := "/tmp/somefile"
 		res.Path = p
-		res.State = "absent"
+		res.State = FileStateAbsent
 
 		timeline := []func() error{
 			fileRemove(p), // nothing here
@@ -936,14 +936,14 @@ func TestResources2(t *testing.T) {
 	}
 	{
 		//file "/tmp/somefile" {
-		//	state => "absent",
+		//	state => $const.res.file.state.absent,
 		//}
 		// and a file already exists!
 		r1 := makeRes("file", "r1")
 		res := r1.(*FileRes) // if this panics, the test will panic
 		p := "/tmp/somefile"
 		res.Path = p
-		res.State = "absent"
+		res.State = FileStateAbsent
 
 		timeline := []func() error{
 			fileWrite(p, "whatever"),
@@ -966,7 +966,7 @@ func TestResources2(t *testing.T) {
 	{
 		//file "/tmp/somefile" {
 		//	content => "some new text\n",
-		//	state => "exists",
+		//	state => $const.res.file.state.exists,
 		//
 		//	Meta:reverse => true,
 		//}
@@ -974,7 +974,7 @@ func TestResources2(t *testing.T) {
 		res := r1.(*FileRes) // if this panics, the test will panic
 		p := "/tmp/somefile"
 		res.Path = p
-		res.State = "exists"
+		res.State = FileStateExists
 		content := "some new text\n"
 		res.Content = &content
 		original := "this is the original state\n" // original state
@@ -1035,7 +1035,7 @@ func TestResources2(t *testing.T) {
 		res := r1.(*FileRes) // if this panics, the test will panic
 		p := "/tmp/somefile"
 		res.Path = p
-		//res.State = "exists" // unspecified
+		//res.State = FileStateExists // unspecified
 		content := "some new text\n"
 		res.Content = &content
 		original := "this is the original state\n" // original state
@@ -1100,7 +1100,7 @@ func TestResources2(t *testing.T) {
 		res := r1.(*FileRes) // if this panics, the test will panic
 		p := "/tmp/somefile"
 		res.Path = p
-		//res.State = "exists" // unspecified
+		//res.State = FileStateExists // unspecified
 		content := "some new text\n"
 		res.Content = &content
 		var r2 engine.Res // future reversed resource
@@ -1149,7 +1149,7 @@ func TestResources2(t *testing.T) {
 	}
 	{
 		//file "/tmp/somefile" {
-		//	state => "absent",
+		//	state => $const.res.file.state.absent,
 		//
 		//	Meta:reverse => true,
 		//}
@@ -1157,7 +1157,7 @@ func TestResources2(t *testing.T) {
 		res := r1.(*FileRes) // if this panics, the test will panic
 		p := "/tmp/somefile"
 		res.Path = p
-		res.State = "absent"
+		res.State = FileStateAbsent
 		original := "this is the original state\n" // original state
 		var r2 engine.Res                          // future reversed resource
 
@@ -1207,7 +1207,7 @@ func TestResources2(t *testing.T) {
 	}
 	{
 		//file "/tmp/somefile" {
-		//	state => "exists",
+		//	state => $const.res.file.state.exists,
 		//	fragments => [
 		//		"/tmp/frag1",
 		//		"/tmp/fragdir1/",
@@ -1220,7 +1220,7 @@ func TestResources2(t *testing.T) {
 		res := r1.(*FileRes) // if this panics, the test will panic
 		p := "/tmp/somefile"
 		res.Path = p
-		res.State = "exists"
+		res.State = FileStateExists
 		res.Fragments = []string{
 			"/tmp/frag1",
 			"/tmp/fragdir1/",
@@ -1272,7 +1272,7 @@ func TestResources2(t *testing.T) {
 	}
 	{
 		//file "/tmp/somefile" {
-		//	state => "exists",
+		//	state => $const.res.file.state.exists,
 		//	source => "/tmp/somefiletocopy",
 		//}
 		r1 := makeRes("file", "r1")
@@ -1281,7 +1281,7 @@ func TestResources2(t *testing.T) {
 		p2 := "/tmp/somefiletocopy"
 		content := "hello this is some file to copy\n"
 		res.Path = p
-		res.State = "exists"
+		res.State = FileStateExists
 		res.Source = p2
 
 		timeline := []func() error{
@@ -1308,13 +1308,13 @@ func TestResources2(t *testing.T) {
 	}
 	{
 		//file "/tmp/somedir/" {
-		//	state => "exists",
+		//	state => $const.res.file.state.exists,
 		//}
 		r1 := makeRes("file", "r1")
 		res := r1.(*FileRes) // if this panics, the test will panic
 		p := "/tmp/somedir/"
 		res.Path = p
-		res.State = "exists"
+		res.State = FileStateExists
 
 		timeline := []func() error{
 			fileAbsent(p), // ensure it's absent
@@ -1337,7 +1337,7 @@ func TestResources2(t *testing.T) {
 	}
 	{
 		//file "/tmp/somedir/" {
-		//	state => "exists",
+		//	state => $const.res.file.state.exists,
 		//	source => /tmp/somedirtocopy/,
 		//	recurse => true,
 		//}
@@ -1346,7 +1346,7 @@ func TestResources2(t *testing.T) {
 		p := "/tmp/somedir/"
 		p2 := "/tmp/somedirtocopy/"
 		res.Path = p
-		res.State = "exists"
+		res.State = FileStateExists
 		res.Source = p2
 		res.Recurse = true
 
@@ -1408,7 +1408,7 @@ func TestResources2(t *testing.T) {
 	}
 	{
 		//file "/tmp/somedir/" {
-		//	state => "exists",
+		//	state => $const.res.file.state.exists,
 		//	recurse => true,
 		//	purge => true,
 		//}
@@ -1416,7 +1416,7 @@ func TestResources2(t *testing.T) {
 		res := r1.(*FileRes) // if this panics, the test will panic
 		p := "/tmp/somedir/"
 		res.Path = p
-		res.State = "exists"
+		res.State = FileStateExists
 		res.Recurse = true
 		res.Purge = true
 
@@ -1474,7 +1474,7 @@ func TestResources2(t *testing.T) {
 	}
 	{
 		//file "/tmp/somedir/" {
-		//	state => "exists",
+		//	state => $const.res.file.state.exists,
 		//	recurse => true,
 		//	purge => true,
 		//}
@@ -1489,7 +1489,7 @@ func TestResources2(t *testing.T) {
 		res := r1.(*FileRes) // if this panics, the test will panic
 		p := "/tmp/somedir/"
 		res.Path = p
-		res.State = "exists"
+		res.State = FileStateExists
 		res.Recurse = true
 		res.Purge = true
 

examples/lang/autoedges1.mcl

@@ -8,14 +8,14 @@ pkg "drbd-utils" {
 
 file "/etc/drbd.conf" {
 	content => "this is an mgmt test",
-	state => "exists",
+	state => $const.res.file.state.exists,
 
 	Meta:autoedge => true,
 	Meta:noop => $noop,
 }
 
 file "/etc/drbd.d/" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 
 	Meta:autoedge => true,
 	Meta:noop => $noop,

examples/lang/contains0.mcl

@@ -11,7 +11,7 @@ $c4 = "b" in $set
 $s = fmt.printf("1: %t, 2: %t, 3: %t, 4: %t\n", $c1, $c2, $c3, $c4)
 
 file "/tmp/mgmt/contains" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => $s,
 }
 
@@ -22,6 +22,6 @@ $x = if sys.hostname() in ["h1", "h3",] {
 }
 
 file "/tmp/mgmt/hello-${sys.hostname()}" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => $x,
 }

examples/lang/cron2.mcl

@@ -5,5 +5,5 @@ cron "purpleidea-oneshot" {
 
 svc "purpleidea-oneshot" {}
 
-# TODO: do we need a state => "exists" specified here?
+# TODO: do we need a state => $const.res.file.state.exists specified here?
 file "/etc/systemd/system/purpleidea-oneshot.service" {}

examples/lang/cron3.mcl

@@ -10,5 +10,5 @@ svc "purpleidea-oneshot" {
 	session => true,
 }
 
-# TODO: do we need a state => "exists" specified here?
+# TODO: do we need a state => $const.res.file.state.exists specified here?
 file printf("%s/.config/systemd/user/purpleidea-oneshot.service", $home) {}

examples/lang/cron4.mcl

@@ -13,5 +13,5 @@ svc "purpleidea-oneshot" {
 }
 
 file printf("%s/.config/systemd/user/purpleidea-oneshot.service", $home) {
-	state => "absent",
+	state => $const.res.file.state.absent,
 }

examples/lang/datetime1.mcl

@@ -2,6 +2,6 @@ import "datetime"
 
 $d = datetime.now()
 file "/tmp/mgmt/datetime" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => template("Hello! It is now: {{ datetime_print . }}\n", $d),
 }

examples/lang/datetime2.mcl

@@ -12,7 +12,7 @@ $theload = structlookup(sys.load(), "x1")
 
 if 5 > 3 {
 	file "/tmp/mgmt/datetime" {
-		state => "exists",
+		state => $const.res.file.state.exists,
 		content => template("Now + 1 year is: {{ .year }} seconds, aka: {{ datetime_print .year }}\n\nload average: {{ .load }}\n", $tmplvalues),
 	}
 }

examples/lang/datetime3.mcl

@@ -14,6 +14,6 @@ $theload = structlookup(sys.load(), "x1")
 $vumeter = example.vumeter("====", 10, 0.9)
 
 file "/tmp/mgmt/datetime" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => template("Now + 1 year is: {{ .year }} seconds, aka: {{ datetime_print .year }}\n\nload average: {{ .load }}\n\nvu: {{ .vumeter }}\n", $tmplvalues),
 }

examples/lang/datetime4.mcl

@@ -5,6 +5,6 @@ import "example"
 $now = datetime.now()
 
 file "/tmp/mgmt-datetime" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => template("Il est l'or Monseignor: {{ . }}\n", datetime.format($now, "15:04:05")),
 }

examples/lang/elvis0.mcl

@@ -1,5 +1,5 @@
 $b = true # change me to false and then try editing the file manually
 file "/tmp/mgmt-elvis" {
 	content => $b ?: "hello world\n",
-	state => "exists",
+	state => $const.res.file.state.exists,
 }

examples/lang/exchange0.mcl

@@ -13,6 +13,6 @@ $rand = random1(8)
 $exchanged = world.exchange("keyns", $rand)
 
 file "/tmp/mgmt/exchange-${sys.hostname()}" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => template("Found: {{ . }}\n", $exchanged),
 }

examples/lang/exec0.mcl

@@ -5,7 +5,7 @@ exec "exec1" {
 }
 
 file "/tmp/whatever" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 
 exec "exec2" {

examples/lang/file0.mcl

@@ -1,7 +1,7 @@
 file "/tmp/file1" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 
 file "/tmp/dir1/" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 }

examples/lang/frag1.mcl

@@ -1,31 +1,31 @@
 file "/tmp/frags/" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 
 # fragments
 file "/tmp/frags/f1" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => "i am f1\n",
 }
 file "/tmp/frags/f2" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => "i am f2\n",
 }
 file "/tmp/frags/f3" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => "i am f3\n",
 }
 
 # You can also drop in an unmanaged file into the frags dir for it to get used!
 # And of course you can hard-code the list of files to use like this one is...
 file "/tmp/bonus_file" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => "i am a bonus file\n",
 }
 
 # automatic edges will get added!
 file "/tmp/whole1" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	fragments => [
 		"/tmp/frags/", # pull from this dir
 		"/tmp/bonus_file", # also pull this one file

examples/lang/hello0.mcl

@@ -1,4 +1,4 @@
 file "/tmp/mgmt-hello-world" {
 	content => "hello world from @purpleidea\n",
-	state => "exists",
+	state => $const.res.file.state.exists,
 }

examples/lang/history1.mcl

@@ -5,6 +5,6 @@ $dt = datetime.now()
 $hystvalues = {"ix0" => $dt, "ix1" => $dt{1}, "ix2" => $dt{2}, "ix3" => $dt{3},}
 
 file "/tmp/mgmt/history" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => template("Index(0) {{.ix0}}: {{ datetime_print .ix0 }}\nIndex(1) {{.ix1}}: {{ datetime_print .ix1 }}\nIndex(2) {{.ix2}}: {{ datetime_print .ix2 }}\nIndex(3) {{.ix3}}: {{ datetime_print .ix3 }}\n", $hystvalues),
 }

examples/lang/hostname0.mcl

@@ -2,5 +2,5 @@ import "sys"
 
 file "/tmp/mgmt/${sys.hostname()}" {
 	content => "hello from ${sys.hostname()}!\n",
-	state => "exists",
+	state => $const.res.file.state.exists,
 }

examples/lang/html.mcl

@@ -5,6 +5,6 @@ $text1 = html.unescape_string("<h1>MGMT!</h1>")
 $text2 = html.escape_string("Test & Re-Test\n")
 
 file "/tmp/index.html" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => "${text1}${text2}",
 }

examples/lang/hysteresis1.mcl

@@ -1,7 +1,7 @@
 import "sys"
 
 file "/tmp/mgmt/systemload" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => template("load average: {{ .load }} threshold: {{ .threshold }}\n", $tmplvalues),
 }
 

examples/lang/interpolate1.mcl

@@ -1,5 +1,5 @@
 $audience = "WORLD!"
 file "/tmp/mgmt/hello" {
 	content => "hello ${audience}!\n",
-	state => "exists",
+	state => $const.res.file.state.exists,
 }

examples/lang/os.mcl

@@ -5,23 +5,23 @@ import "fmt"
 $tmpdir = os.temp_dir()
 
 file "${tmpdir}/execinfo" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => fmt.printf("mgmt is at %s\n", os.executable()),
 }
 
 file "${tmpdir}/mgmtenv" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => os.expand_env("$HOME sweet ${os.getenv(\"HOME\")}\n"),
 }
 
 file "${tmpdir}/mgmtos" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => os.readlink("/bin"),
 }
 
 $rm = exec.look_path("rm")
 
 file "${tmpdir}/cache" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => "Plz cache in ${os.user_cache_dir()}.\nYour home is ${os.user_home_dir()}. Remove with ${rm}\n",
 }

examples/lang/password0.mcl

@@ -3,7 +3,7 @@ password "pass0" {
 }
 
 file "/tmp/mgmt/password" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 
 Password["pass0"].password -> File["/tmp/mgmt/password"].content

examples/lang/purge1.mcl

@@ -1,19 +1,19 @@
 file "/tmp/some_dir/" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	#source => "", # this default means empty directory
 	recurse => true,
 	purge => true,
 }
 
 file "/tmp/some_dir/fileA" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => "i am fileA\n",
 }
 file "/tmp/some_dir/fileB" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => "i am fileB\n",
 }
 file "/tmp/some_dir/fileC" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => "i am fileC\n",
 }

examples/lang/readfile1.mcl

@@ -2,6 +2,6 @@ import "os"
 
 # this copies the contents from /tmp/input and puts them in /tmp/output
 file "/tmp/output" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => os.readfile("/tmp/input"),
 }

examples/lang/readonlyfriday.mcl

@@ -21,7 +21,7 @@ print "msg" {
 }
 
 file "/tmp/files/" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	mode => if $is_friday { # this updates the mode, the instant it changes!
 		"0550"
 	} else {

examples/lang/reverse1.mcl

@@ -11,14 +11,14 @@ $mod3 = math.mod($now, 8) == 3
 $mod = $mod0 || $mod1 || $mod2 || $mod3
 
 file "/tmp/mgmt/" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 
 # file should disappear and re-appear every four seconds
 if $mod {
 	file "/tmp/mgmt/hello" {
 		content => "please say abracadabra...\n",
-		state => "exists",
+		state => $const.res.file.state.exists,
 
 		Meta:reverse => true,
 	}

examples/lang/reverse2.mcl

@@ -11,14 +11,14 @@ $mod3 = math.mod($now, 8) == 3
 $mod = $mod0 || $mod1 || $mod2 || $mod3
 
 file "/tmp/mgmt/" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 
 # file should re-appear and disappear every four seconds
 # it will even preserve and then restore the pre-existing content!
 if $mod {
 	file "/tmp/mgmt/hello" {
-		state => "absent", # delete the file
+		state => $const.res.file.state.absent, # delete the file
 
 		Meta:reverse => true,
 	}

examples/lang/reverse3.mcl

@@ -11,7 +11,7 @@ $mod3 = math.mod($now, 8) == 3
 $mod = $mod0 || $mod1 || $mod2 || $mod3
 
 file "/tmp/mgmt/" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 
 # file should change the mode every four seconds
@@ -19,7 +19,7 @@ file "/tmp/mgmt/" {
 # you should create the file before you run this
 if $mod {
 	file "/tmp/mgmt/hello" {
-		#state => "exists",	# omit to see it change the mode only!
+		#state => $const.res.file.state.exists,	# omit to see it change the mode only!
 		mode => "0777",
 
 		Meta:reverse => true,

examples/lang/runtime.mcl

@@ -2,6 +2,6 @@ import "golang/runtime"
 import "fmt"
 
 file "/tmp/mgmtinfo" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => fmt.printf("Hi from mgmt! mgmt is running with %s; and GOROOT is %s.\n", runtime.version(), runtime.goroot()),
 }

examples/lang/schedule0.mcl

@@ -17,6 +17,6 @@ $set = world.schedule("xsched", $opts)
 #$set = world.schedule("xsched")
 
 file "/tmp/mgmt/scheduled-${sys.hostname()}" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => template("set: {{ . }}\n", $set),
 }

examples/lang/sendrecv0.mcl

@@ -4,7 +4,7 @@ exec "exec0" {
 }
 
 file ["/tmp/command-output", "/tmp/command-stdout", "/tmp/command-stderr",] {
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 
 Exec["exec0"].output -> File["/tmp/command-output"].content

examples/lang/sendrecv1.mcl

@@ -19,7 +19,7 @@ Exec["exec0"].output -> Kv["kv0"].value
 
 if $state != "default" {
 	file "/tmp/mgmt/state" {
-		state => "exists",
+		state => $const.res.file.state.exists,
 		content => fmt.printf("state: %s\n", $state),
 	}
 }

examples/lang/states0.mcl

@@ -7,7 +7,7 @@ $state = maplookup($exchanged, $hostname, "default")
 if $state == "one" || $state == "default" {
 
 	file "/tmp/mgmt/state" {
-		state => "exists",
+		state => $const.res.file.state.exists,
 		content => "state: one\n",
 	}
 
@@ -23,7 +23,7 @@ if $state == "one" || $state == "default" {
 if $state == "two" {
 
 	file "/tmp/mgmt/state" {
-		state => "exists",
+		state => $const.res.file.state.exists,
 		content => "state: two\n",
 	}
 
@@ -39,7 +39,7 @@ if $state == "two" {
 if $state == "three" {
 
 	file "/tmp/mgmt/state" {
-		state => "exists",
+		state => $const.res.file.state.exists,
 		content => "state: three\n",
 	}
 

examples/lang/tftp0.mcl

@@ -1,10 +1,10 @@
 $root = "/tmp/tftproot/"
 file $root {
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 file "${root}file0" {
 	content => "i'm file0 in ${root}\n",
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 
 tftp:server ":1069" {	# by default tftp uses :69 but using :1069 avoids needing root!
@@ -22,7 +22,7 @@ tftp:file "/file1" {
 $f2 = "/tmp/some_file"
 file $f2 {
 	content => "i'm a cool file in /tmp\n",
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 
 # you can point to it directly...

examples/lang/unicode.mcl

@@ -3,6 +3,6 @@ print "unicode" {
 	msg => $unicode,
 }
 file "/tmp/unicode" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => $unicode + "\n",
 }

examples/lang/virt2.mcl

@@ -17,7 +17,7 @@ $count = if $input > 8 {
 }
 
 file "/tmp/output" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => fmt.printf("requesting: %d cpus\n", $count),
 }
 

integration/basic_test.go

@@ -37,7 +37,7 @@ func TestInstance0(t *testing.T) {
 
 	file "${root}/mgmt-hello-world" {
 		content => "hello world from @purpleidea\n",
-		state => "exists",
+		state => $const.res.file.state.exists,
 	}
 	`
 	m := Instance{
@@ -82,7 +82,7 @@ func TestInstance1(t *testing.T) {
 
 		file "${root}/mgmt-hello-world" {
 			content => "hello world from @purpleidea\n",
-			state => "exists",
+			state => $const.res.file.state.exists,
 		}
 		`)
 		testCases = append(testCases, test{
@@ -170,7 +170,7 @@ func TestCluster1(t *testing.T) {
 
 		file "${root}/mgmt-hostname" {
 			content => "i am ${sys.hostname()}\n",
-			state => "exists",
+			state => $const.res.file.state.exists,
 		}
 		`)
 		testCases = append(testCases, test{
@@ -195,7 +195,7 @@ func TestCluster1(t *testing.T) {
 
 		file "${root}/mgmt-hostname" {
 			content => "i am ${sys.hostname()}\n",
-			state => "exists",
+			state => $const.res.file.state.exists,
 		}
 		`)
 		testCases = append(testCases, test{

lang/funcs/vars/vars.go

@@ -0,0 +1,136 @@
+// Mgmt
+// Copyright (C) 2013-2020+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+// Package vars provides a framework for language vars.
+package vars
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/purpleidea/mgmt/lang/interfaces"
+)
+
+const (
+	// ConstNamespace is the string prefix for all top-level built-in vars.
+	ConstNamespace = "const"
+
+	// ResourceNamespace is the string prefix for all top-level resource
+	// specific built-in vars, that exist under the ConstNamespace header.
+	ResourceNamespace = "res"
+)
+
+// registeredVars is a global map of all possible vars which can be used. You
+// should never touch this map directly. Use methods like Register instead.
+var registeredVars = make(map[string]func() interfaces.Var) // must initialize
+
+// Register takes a var and its name and makes it available for use. It is
+// commonly called in the init() method of the var at program startup. There is
+// no matching Unregister function.
+func Register(name string, fn func() interfaces.Var) {
+	if _, ok := registeredVars[name]; ok {
+		panic(fmt.Sprintf("a var named %s is already registered", name))
+	}
+	//gob.Register(fn())
+	registeredVars[name] = fn
+}
+
+// ModuleRegister is exactly like Register, except that it registers within a
+// named module. This is a helper function.
+//func ModuleRegister(module, name string, v func() interfaces.Var) {
+//	Register(module+interfaces.ModuleSep+name, v)
+//}
+
+// resourceConstHelper is a helper function to manage the const topology.
+func resourceConstHelper(kind, param, field string) string {
+	// const.res.file.state.exists = "exists"
+	// TODO: should it be: const.res.file.params.state.exists = "exists" ?
+	chunks := []string{
+		ConstNamespace,
+		ResourceNamespace,
+		kind,
+		param,
+		field,
+	}
+	//return ConstNamespace + interfaces.ModuleSep + ResourceNamespace + interfaces.ModuleSep + kind + interfaces.ModuleSep + param + interfaces.ModuleSep + field
+	return strings.Join(chunks, interfaces.ModuleSep) // cleaner code
+}
+
+// RegisterResourceParam registers a single const param for a resource. You
+// might prefer to use RegisterResourceParams instead.
+func RegisterResourceParam(kind, param, field string, value func() interfaces.Var) {
+	Register(resourceConstHelper(kind, param, field), value)
+}
+
+// RegisterResourceParams registers a map of const params for a resource. The
+// params mapping keys are the param name and the param field name. Finally, the
+// value is the specific type value for that constant.
+func RegisterResourceParams(kind string, params map[string]map[string]func() interfaces.Var) {
+	for param, mapping := range params {
+		for field, value := range mapping {
+			Register(resourceConstHelper(kind, param, field), value)
+		}
+	}
+}
+
+// Lookup returns a pointer to the var implementation.
+func Lookup(name string) (interfaces.Var, error) {
+	f, exists := registeredVars[name]
+	if !exists {
+		return nil, fmt.Errorf("not found")
+	}
+	return f(), nil
+}
+
+// LookupPrefix returns a map of names to vars that start with a module prefix.
+// This search automatically adds the period separator. So if you want vars in
+// the `const` prefix, search for `const`, not `const.` and it will find all the
+// correctly registered vars. This removes that prefix from the result in the
+// map keys that it returns. If you search for an empty prefix, then this will
+// return all the top-level functions that aren't in a module.
+func LookupPrefix(prefix string) map[string]func() interfaces.Var {
+	result := make(map[string]func() interfaces.Var)
+	for name, f := range registeredVars {
+		// requested top-level vars, and no module separators...
+		if prefix == "" {
+			if !strings.Contains(name, interfaces.ModuleSep) {
+				result[name] = f // copy
+			}
+			continue
+		}
+		sep := prefix + interfaces.ModuleSep
+		if !strings.HasPrefix(name, sep) {
+			continue
+		}
+		s := strings.TrimPrefix(name, sep) // remove the prefix
+		result[s] = f                      // copy
+	}
+	return result
+}
+
+// Map returns a map from all registered var names to a function to return that
+// one. We return a copy of our internal registered var store so that this
+// result can be manipulated safely. We return the vars that produce the Var
+// interface because we might use this result to create multiple vars, and each
+// one might need to have its own unique memory address to work properly.
+func Map() map[string]func() interfaces.Var {
+	m := make(map[string]func() interfaces.Var)
+	for name, fn := range registeredVars { // copy
+		m[name] = fn
+	}
+	return m
+}

lang/gapi.go

@@ -24,6 +24,7 @@ import (
 	"sync"
 
 	"github.com/purpleidea/mgmt/gapi"
+	"github.com/purpleidea/mgmt/lang/funcs/vars"
 	"github.com/purpleidea/mgmt/lang/interfaces"
 	"github.com/purpleidea/mgmt/lang/unification"
 	"github.com/purpleidea/mgmt/pgraph"
@@ -264,13 +265,21 @@ func (obj *GAPI) Cli(cliInfo *gapi.CliInfo) (*gapi.Deploy, error) {
 		return nil, errwrap.Wrapf(err, "could not interpolate AST")
 	}
 
+	variables := map[string]interfaces.Expr{
+		"purpleidea": &ExprStr{V: "hello world!"}, // james says hi
+		// TODO: change to a func when we can change hostname dynamically!
+		"hostname": &ExprStr{V: ""}, // NOTE: empty b/c not used
+	}
+	consts := VarPrefixToVariablesScope(vars.ConstNamespace) // strips prefix!
+	addback := vars.ConstNamespace + interfaces.ModuleSep    // add it back...
+	variables, err = MergeExprMaps(variables, consts, addback)
+	if err != nil {
+		return nil, errwrap.Wrapf(err, "couldn't merge in consts")
+	}
+
 	// top-level, built-in, initial global scope
 	scope := &interfaces.Scope{
-		Variables: map[string]interfaces.Expr{
-			"purpleidea": &ExprStr{V: "hello world!"}, // james says hi
-			// TODO: change to a func when we can change hostname dynamically!
-			"hostname": &ExprStr{V: ""}, // NOTE: empty b/c not used
-		},
+		Variables: variables,
 		// all the built-in top-level, core functions enter here...
 		Functions: FuncPrefixToFunctionsScope(""), // runs funcs.LookupPrefix
 	}

lang/interfaces/var.go

@@ -0,0 +1,28 @@
+// Mgmt
+// Copyright (C) 2013-2020+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package interfaces
+
+import (
+	"github.com/purpleidea/mgmt/lang/types"
+)
+
+// Var is the interface that any valid stand-alone variable must fulfill.
+// NOTE: It's just a simple Value from our types library at the moment.
+type Var interface {
+	types.Value
+}

lang/interpret_test.go

@@ -33,6 +33,7 @@ import (
 	"github.com/purpleidea/mgmt/engine/resources"
 	"github.com/purpleidea/mgmt/etcd"
 	"github.com/purpleidea/mgmt/lang/funcs"
+	"github.com/purpleidea/mgmt/lang/funcs/vars"
 	"github.com/purpleidea/mgmt/lang/interfaces"
 	"github.com/purpleidea/mgmt/lang/unification"
 	"github.com/purpleidea/mgmt/pgraph"
@@ -555,12 +556,22 @@ func TestAstFunc1(t *testing.T) {
 		return
 	}
 	t.Logf("tests directory is: %s", dir)
+
+	variables := map[string]interfaces.Expr{
+		"purpleidea": &ExprStr{V: "hello world!"}, // james says hi
+		// TODO: change to a func when we can change hostname dynamically!
+		"hostname": &ExprStr{V: ""}, // NOTE: empty b/c not used
+	}
+	consts := VarPrefixToVariablesScope(vars.ConstNamespace) // strips prefix!
+	addback := vars.ConstNamespace + interfaces.ModuleSep    // add it back...
+	variables, err = MergeExprMaps(variables, consts, addback)
+	if err != nil {
+		t.Errorf("couldn't merge in consts: %+v", err)
+		return
+	}
+
 	scope := &interfaces.Scope{ // global scope
-		Variables: map[string]interfaces.Expr{
-			"purpleidea": &ExprStr{V: "hello world!"}, // james says hi
-			// TODO: change to a func when we can change hostname dynamically!
-			"hostname": &ExprStr{V: ""}, // NOTE: empty b/c not used
-		},
+		Variables: variables,
 		// all the built-in top-level, core functions enter here...
 		Functions: FuncPrefixToFunctionsScope(""), // runs funcs.LookupPrefix
 	}
@@ -977,12 +988,22 @@ func TestAstFunc2(t *testing.T) {
 		return
 	}
 	t.Logf("tests directory is: %s", dir)
+
+	variables := map[string]interfaces.Expr{
+		"purpleidea": &ExprStr{V: "hello world!"}, // james says hi
+		// TODO: change to a func when we can change hostname dynamically!
+		"hostname": &ExprStr{V: ""}, // NOTE: empty b/c not used
+	}
+	consts := VarPrefixToVariablesScope(vars.ConstNamespace) // strips prefix!
+	addback := vars.ConstNamespace + interfaces.ModuleSep    // add it back...
+	variables, err = MergeExprMaps(variables, consts, addback)
+	if err != nil {
+		t.Errorf("couldn't merge in consts: %+v", err)
+		return
+	}
+
 	scope := &interfaces.Scope{ // global scope
-		Variables: map[string]interfaces.Expr{
-			"purpleidea": &ExprStr{V: "hello world!"}, // james says hi
-			// TODO: change to a func when we can change hostname dynamically!
-			"hostname": &ExprStr{V: ""}, // NOTE: empty b/c not used
-		},
+		Variables: variables,
 		// all the built-in top-level, core functions enter here...
 		Functions: FuncPrefixToFunctionsScope(""), // runs funcs.LookupPrefix
 	}

lang/interpret_test/TestAstFunc2/const1.output

@@ -0,0 +1 @@
+Vertex: test[exists]

lang/interpret_test/TestAstFunc2/const1/main.mcl

@@ -0,0 +1,2 @@
+# this magic string variable should exist and be "exists"
+test $const.res.file.state.exists {}

lang/interpret_test/TestAstFunc2/file-fragments1/main.mcl

@@ -1,31 +1,31 @@
 file "/tmp/frags/" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 
 # fragments
 file "/tmp/frags/f1" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => "i am f1\n",
 }
 file "/tmp/frags/f2" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => "i am f2\n",
 }
 file "/tmp/frags/f3" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => "i am f3\n",
 }
 
 # You can also drop in an unmanaged file into the frags dir for it to get used!
 # And of course you can hard-code the list of files to use like this one is...
 file "/tmp/bonus_file" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => "i am a bonus file\n",
 }
 
 # automatic edges will get added!
 file "/tmp/whole1" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	fragments => [
 		"/tmp/frags/", # pull from this dir
 		"/tmp/bonus_file", # also pull this one file

lang/interpret_test/TestAstFunc2/send-recv-0/main.mcl

@@ -4,7 +4,7 @@ exec "exec0" {
 }
 
 file ["/tmp/command-output", "/tmp/command-stdout", "/tmp/command-stderr",] {
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 
 Exec["exec0"].output -> File["/tmp/command-output"].content

lang/interpret_test/TestAstFunc2/send-recv-1/main.mcl

@@ -4,7 +4,7 @@ exec "exec0" {
 }
 
 file "/tmp/command-output" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 
 # this is an error because the shell send key doesn't exist in exec

lang/interpret_test/TestAstFunc2/send-recv-2/main.mcl

@@ -4,7 +4,7 @@ exec ["exec0", "exec1",] {
 }
 
 file "/tmp/command-output" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 
 # this is an error because two senders cannot send to the same receiver key

lang/lang.go

@@ -25,6 +25,7 @@ import (
 	"github.com/purpleidea/mgmt/engine"
 	"github.com/purpleidea/mgmt/lang/funcs"
 	_ "github.com/purpleidea/mgmt/lang/funcs/core" // import so the funcs register
+	"github.com/purpleidea/mgmt/lang/funcs/vars"
 	"github.com/purpleidea/mgmt/lang/interfaces"
 	"github.com/purpleidea/mgmt/lang/unification"
 	"github.com/purpleidea/mgmt/pgraph"
@@ -165,13 +166,21 @@ func (obj *Lang) Init() error {
 	}
 	obj.ast = interpolated
 
+	variables := map[string]interfaces.Expr{
+		"purpleidea": &ExprStr{V: "hello world!"}, // james says hi
+		// TODO: change to a func when we can change hostname dynamically!
+		"hostname": &ExprStr{V: obj.Hostname},
+	}
+	consts := VarPrefixToVariablesScope(vars.ConstNamespace) // strips prefix!
+	addback := vars.ConstNamespace + interfaces.ModuleSep    // add it back...
+	variables, err = MergeExprMaps(variables, consts, addback)
+	if err != nil {
+		return errwrap.Wrapf(err, "couldn't merge in consts")
+	}
+
 	// top-level, built-in, initial global scope
 	scope := &interfaces.Scope{
-		Variables: map[string]interfaces.Expr{
-			"purpleidea": &ExprStr{V: "hello world!"}, // james says hi
-			// TODO: change to a func when we can change hostname dynamically!
-			"hostname": &ExprStr{V: obj.Hostname},
-		},
+		Variables: variables,
 		// all the built-in top-level, core functions enter here...
 		Functions: FuncPrefixToFunctionsScope(""), // runs funcs.LookupPrefix
 	}

lang/structs.go

@@ -3073,9 +3073,10 @@ func (obj *StmtProg) importSystemScope(name string) (*interfaces.Scope, error) {
 
 	// initial scope, built from core golang code
 	scope := &interfaces.Scope{
-		// TODO: we could add core API's for variables and classes too!
+		// TODO: we could use the core API for variables somehow...
 		//Variables: make(map[string]interfaces.Expr),
-		Functions: funcs, // map[string]Expr
+		Functions: funcs, // map[string]interfaces.Expr
+		// TODO: we could add a core API for classes too!
 		//Classes: make(map[string]interfaces.Stmt),
 	}
 

lang/unification_test.go

@@ -24,6 +24,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/purpleidea/mgmt/lang/funcs/vars"
 	"github.com/purpleidea/mgmt/lang/interfaces"
 	"github.com/purpleidea/mgmt/lang/types"
 	"github.com/purpleidea/mgmt/lang/unification"
@@ -858,12 +859,23 @@ func TestUnification1(t *testing.T) {
 			//}
 			//t.Logf("test #%d: astInterpolated: %+v", index, astInterpolated)
 
+			variables := map[string]interfaces.Expr{
+				"purpleidea": &ExprStr{V: "hello world!"}, // james says hi
+				//"hostname": &ExprStr{V: obj.Hostname},
+			}
+			consts := VarPrefixToVariablesScope(vars.ConstNamespace) // strips prefix!
+			addback := vars.ConstNamespace + interfaces.ModuleSep    // add it back...
+			var err error
+			variables, err = MergeExprMaps(variables, consts, addback)
+			if err != nil {
+				t.Errorf("test #%d: FAIL", index)
+				t.Errorf("test #%d: couldn't merge in consts: %+v", index, err)
+				return
+			}
+
 			// top-level, built-in, initial global scope
 			scope := &interfaces.Scope{
-				Variables: map[string]interfaces.Expr{
-					"purpleidea": &ExprStr{V: "hello world!"}, // james says hi
-					//"hostname": &ExprStr{V: obj.Hostname},
-				},
+				Variables: variables,
 				// all the built-in top-level, core functions enter here...
 				Functions: FuncPrefixToFunctionsScope(""), // runs funcs.LookupPrefix
 			}
@@ -884,7 +896,7 @@ func TestUnification1(t *testing.T) {
 				Debug:  testing.Verbose(),
 				Logf:   logf,
 			}
-			err := unifier.Unify()
+			err = unifier.Unify()
 
 			// TODO: print out the AST's so that we can see the types
 			t.Logf("\n\ntest #%d: AST (after): %+v\n", index, ast)

lang/util.go

@@ -18,9 +18,13 @@
 package lang
 
 import (
+	"fmt"
+	"strings"
+
 	"github.com/purpleidea/mgmt/lang/funcs"
 	"github.com/purpleidea/mgmt/lang/funcs/simple"
 	"github.com/purpleidea/mgmt/lang/funcs/simplepoly"
+	"github.com/purpleidea/mgmt/lang/funcs/vars"
 	"github.com/purpleidea/mgmt/lang/interfaces"
 	"github.com/purpleidea/mgmt/lang/types"
 )
@@ -43,6 +47,7 @@ func FuncPrefixToFunctionsScope(prefix string) map[string]interfaces.Expr {
 
 				Values: []*types.FuncValue{st.Fn}, // just one!
 			}
+			// XXX: should we run fn.SetType(st.Fn.Type()) ?
 			exprs[name] = fn
 			continue
 		} else if st, ok := x.(*simplepoly.WrappedFunc); simplepoly.DirectInterface && ok {
@@ -66,3 +71,147 @@ func FuncPrefixToFunctionsScope(prefix string) map[string]interfaces.Expr {
 	}
 	return exprs
 }
+
+// VarPrefixToVariablesScope is a helper function to return the variables
+// portion of the scope from a variable prefix lookup. Basically this is useful
+// to pull out a portion of the variables we've defined by API.
+func VarPrefixToVariablesScope(prefix string) map[string]interfaces.Expr {
+	fns := vars.LookupPrefix(prefix) // map[string]func() interfaces.Var
+	exprs := make(map[string]interfaces.Expr)
+	for name, f := range fns {
+		x := f() // inspect
+		expr, err := ValueToExpr(x)
+		if err != nil {
+			panic(fmt.Sprintf("could not build expr: %+v", err))
+		}
+		exprs[name] = expr
+	}
+	return exprs
+}
+
+// MergeExprMaps merges the two maps of Expr's, and errors if any overwriting
+// would occur. If any prefix string is specified, that is added to the keys of
+// the second "extra" map before doing the merge. This doesn't change the input
+// maps.
+func MergeExprMaps(m, extra map[string]interfaces.Expr, prefix ...string) (map[string]interfaces.Expr, error) {
+	p := strings.Join(prefix, "") // hack to have prefix be optional
+
+	result := map[string]interfaces.Expr{}
+	for k, v := range m {
+		result[k] = v // copy
+	}
+
+	for k, v := range extra {
+		name := p + k
+		if _, exists := result[name]; exists {
+			return nil, fmt.Errorf("duplicate variable: %s", name)
+		}
+		result[name] = v
+	}
+
+	return result, nil
+}
+
+// ValueToExpr converts a Value into the equivalent Expr.
+// FIXME: Add some tests for this function.
+func ValueToExpr(val types.Value) (interfaces.Expr, error) {
+	var expr interfaces.Expr
+
+	switch x := val.(type) {
+	case *types.BoolValue:
+		expr = &ExprBool{
+			V: x.Bool(),
+		}
+
+	case *types.StrValue:
+		expr = &ExprStr{
+			V: x.Str(),
+		}
+
+	case *types.IntValue:
+		expr = &ExprInt{
+			V: x.Int(),
+		}
+
+	case *types.FloatValue:
+		expr = &ExprFloat{
+			V: x.Float(),
+		}
+
+	case *types.ListValue:
+		exprs := []interfaces.Expr{}
+
+		for _, v := range x.List() {
+			e, err := ValueToExpr(v)
+			if err != nil {
+				return nil, err
+			}
+			exprs = append(exprs, e)
+		}
+
+		expr = &ExprList{
+			Elements: exprs,
+		}
+
+	case *types.MapValue:
+		kvs := []*ExprMapKV{}
+
+		for k, v := range x.Map() {
+			kx, err := ValueToExpr(k)
+			if err != nil {
+				return nil, err
+			}
+			vx, err := ValueToExpr(v)
+			if err != nil {
+				return nil, err
+			}
+			kv := &ExprMapKV{
+				Key: kx,
+				Val: vx,
+			}
+			kvs = append(kvs, kv)
+		}
+
+		expr = &ExprMap{
+			KVs: kvs,
+		}
+
+	case *types.StructValue:
+		fields := []*ExprStructField{}
+
+		for k, v := range x.Struct() {
+			fx, err := ValueToExpr(v)
+			if err != nil {
+				return nil, err
+			}
+			field := &ExprStructField{
+				Name:  k,
+				Value: fx,
+			}
+			fields = append(fields, field)
+		}
+
+		expr = &ExprStruct{
+			Fields: fields,
+		}
+
+	case *types.FuncValue:
+		// TODO: this particular case is particularly untested!
+		expr = &ExprFunc{
+			Title: "<func from ValueToExpr>", // TODO: change this?
+			// TODO: symmetrically, it would have used x.Func() here
+			Values: []*types.FuncValue{
+				x, // just one!
+			},
+		}
+
+	case *types.VariantValue:
+		// TODO: should this be allowed, or should we unwrap them?
+		return nil, fmt.Errorf("variant values are not supported")
+
+	default:
+		return nil, fmt.Errorf("unknown type (%T) for value: %+v", val, val)
+	}
+
+	return expr, expr.SetType(val.Type())
+}

test/shell/env0.mcl

@@ -18,6 +18,6 @@ $env = sys.env()
 $m = maplookup($env, "TEST", "321")
 
 file "${tmpdir}/environ" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => fmt.printf("%s,%s,%s:%s,%s,%s:%t,%t:%s", $x, $y, $z, $a, $b, $c, $t, $f, $m),
 }

test/shell/exchange0.mcl

@@ -13,6 +13,6 @@ $rand = random1(8)
 $exchanged = world.exchange("keyns", $rand)
 
 file "/tmp/mgmt/exchange-${sys.hostname()}" {
-	state => "exists",
+	state => $const.res.file.state.exists,
 	content => template("Found: {{ . }}\n", $exchanged),
 }

test/shell/file-source.mcl

@@ -1,4 +1,4 @@
 file "/tmp/mgmt/file-source.txt" {
 	source => "file-source.txt",
-	state => "exists",
+	state => $const.res.file.state.exists,
 }

test/shell/load0.sh

@@ -33,7 +33,7 @@ import "sys"
 
 file "${tmpdir}/loadavg" {
 	content => fmt.printf("load average: %f, %f, %f\n", \$x1, \$x5, \$x15),
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 EOF
 

util/code_test.go

@@ -30,7 +30,7 @@ func TestCodeIndent(t *testing.T) {
 
 	file "${root}/mgmt-hello-world" {
 		content => "hello world from @purpleidea\n",
-		state => "exists",
+		state => $const.res.file.state.exists,
 	}
 	`)
 	c2 :=
@@ -38,7 +38,7 @@ func TestCodeIndent(t *testing.T) {
 
 file "${root}/mgmt-hello-world" {
 	content => "hello world from @purpleidea\n",
-	state => "exists",
+	state => $const.res.file.state.exists,
 }
 `
 	if c1 != c2 {