lang: Add a resource-specific elvis operator

This allows you to omit a resource parameter programmatically, and
avoids the need of an `undef` or `nil` in our language, which would
contribute to programming errors, crashes, and overall reduced safety.

docs/language-guide.md

@@ -90,7 +90,7 @@ There are a very small number of statements in our language. They include:
 - **if**: produces up to one branch of statements based on a conditional
 expression
 
-	```
+	```mcl
 	if <conditional> {
 		<statements>
 	} else {
@@ -101,7 +101,7 @@ expression
 
 - **resource**: produces a resource
 
-	```
+	```mcl
 	file "/tmp/hello" {
 		content => "world",
 		mode => "o=rwx",
@@ -110,7 +110,7 @@ expression
 
 - **edge**: produces an edge
 
-	```
+	```mcl
 	File["/tmp/hello"] -> Print["alert4"]
 	```
 
@@ -131,14 +131,44 @@ This section needs better documentation.
 
 #### Resource
 
-This section needs better documentation.
+Resources express the idempotent workloads that we want to have apply on our
+system. They correspond to vertices in a [graph](https://en.wikipedia.org/wiki/Directed_acyclic_graph)
+which represent the order in which their declared state is applied. You will
+usually want to pass in a number of parameters and associated values to the
+resource to control how it behaves. For example, setting the `content` parameter
+of a `file` resource to the string `hello`, will cause the contents of that file
+to contain the string `hello` after it has run.
+
+For some parameters, there is a distinction between an unspecified parameter,
+and a parameter with a `zero` value. For example, for the file resource, you
+might choose to set the `content` parameter to be the empty string, which would
+ensure that the file has a length of zero. Alternatively you might wish to not
+specify the file contents at all, which would leave that property undefined. If
+you omit listing a property, then it will be undefined. To control this property
+programmatically, you need to specify an `is-defined` value, as well as the
+value to use if that boolean is true. You can do this with the resource-specific
+`elvis` operator.
+
+```mcl
+$b = true # change me to false and then try editing the file manually
+file "/tmp/mgmt-elvis" {
+	content => $b ?: "hello world\n",
+	state => "exists",
+}
+```
+
+This example is static, however you can imagine that the `$b` value might be
+chosen in a programmatic way, even one in which that value varies over time. If
+it evaluates to `true`, then the parameter will be used. If no `elvis` operator
+is specified, then the parameter value will also be used. If the parameter is
+not specified, then it will obviously not be used.
 
 #### Edge
 
 Edges express dependencies in the graph of resources which are output. They can
 be chained as a pair, or in any greater number. For example, you may write:
 
-```
+```mcl
 Pkg["drbd"] -> File["/etc/drbd.conf"] -> Svc["drbd"]
 ```
 

examples/lang/elvis0.mcl

@@ -0,0 +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",
+}

lang/lexer.nex

@@ -39,6 +39,11 @@
 			lval.str = yylex.Text()
 			return ELSE
 		}
+/\?:/		{
+			yylex.pos(lval) // our pos
+			lval.str = yylex.Text()
+			return ELVIS
+		}
 /=>/		{
 			yylex.pos(lval) // our pos
 			lval.str = yylex.Text()

lang/lexparse_test.go

@@ -122,6 +122,20 @@ func TestLexParse0(t *testing.T) {
 			//exp: ???, // FIXME: add the expected AST
 		})
 	}
+	{
+		values = append(values, test{
+			name: "one res with elvis",
+			code: `
+			test "t1" {
+				int16 => true ?: 42, # elvis operator
+				int32 => 42,
+				stringptr => false ?: "", # missing is not ""
+			}
+			`,
+			fail: false,
+			//exp: ???, // FIXME: add the expected AST
+		})
+	}
 	{
 		// TODO: skip trailing comma requirement on one-liners
 		values = append(values, test{

lang/parser.y

@@ -77,7 +77,7 @@ func init() {
 %token STRING BOOL INTEGER FLOAT
 %token EQUALS
 %token COMMA COLON SEMICOLON
-%token ROCKET ARROW DOT
+%token ELVIS ROCKET ARROW DOT
 %token STR_IDENTIFIER BOOL_IDENTIFIER INT_IDENTIFIER FLOAT_IDENTIFIER
 %token STRUCT_IDENTIFIER VARIANT_IDENTIFIER VAR_IDENTIFIER IDENTIFIER
 %token VAR_IDENTIFIER_HX CAPITALIZED_IDENTIFIER
@@ -682,6 +682,11 @@ resource_body:
 		posLast(yylex, yyDollar) // our pos
 		$$.resFields = append($1.resFields, $2.resField)
 	}
+|	resource_body conditional_resource_field
+	{
+		posLast(yylex, yyDollar) // our pos
+		$$.resFields = append($1.resFields, $2.resField)
+	}
 ;
 resource_field:
 	IDENTIFIER ROCKET expr COMMA
@@ -693,6 +698,18 @@ resource_field:
 		}
 	}
 ;
+conditional_resource_field:
+	// content => $present ?: "hello",
+	IDENTIFIER ROCKET expr ELVIS expr COMMA
+	{
+		posLast(yylex, yyDollar) // our pos
+		$$.resField = &StmtResField{
+			Field: $1.str,
+			Value: $5.expr,
+			Condition: $3.expr,
+		}
+	}
+;
 edge:
 	// TODO: we could technically prevent single edge_half pieces from being
 	// parsed, but it's probably more work than is necessary...

lang/structs.go

@@ -120,9 +120,17 @@ func (obj *StmtRes) Interpolate() (interfaces.Stmt, error) {
 		if err != nil {
 			return nil, err
 		}
+		var condition interfaces.Expr
+		if x.Condition != nil {
+			condition, err = x.Condition.Interpolate()
+			if err != nil {
+				return nil, err
+			}
+		}
 		field := &StmtResField{
-			Field: x.Field,
-			Value: interpolated,
+			Field:     x.Field,
+			Value:     interpolated,
+			Condition: condition,
 		}
 		fields = append(fields, field)
 	}
@@ -143,6 +151,11 @@ func (obj *StmtRes) SetScope(scope *interfaces.Scope) error {
 		if err := x.Value.SetScope(scope); err != nil {
 			return err
 		}
+		if x.Condition != nil {
+			if err := x.Condition.SetScope(scope); err != nil {
+				return err
+			}
+		}
 	}
 	return nil
 }
@@ -173,6 +186,22 @@ func (obj *StmtRes) Unify() ([]interfaces.Invariant, error) {
 			return nil, err
 		}
 		invariants = append(invariants, invars...)
+
+		// conditional expression might have some children invariants to share
+		if x.Condition != nil {
+			condition, err := x.Condition.Unify()
+			if err != nil {
+				return nil, err
+			}
+			invariants = append(invariants, condition...)
+
+			// the condition must ultimately be a boolean
+			conditionInvar := &unification.EqualsInvariant{
+				Expr: x.Condition,
+				Type: types.TypeBool,
+			}
+			invariants = append(invariants, conditionInvar)
+		}
 	}
 
 	typMap, err := resources.LangFieldNameToStructType(obj.Kind)
@@ -230,6 +259,14 @@ func (obj *StmtRes) Graph() (*pgraph.Graph, error) {
 			return nil, err
 		}
 		graph.AddGraph(g)
+
+		if x.Condition != nil {
+			g, err := x.Condition.Graph()
+			if err != nil {
+				return nil, err
+			}
+			graph.AddGraph(g)
+		}
 	}
 
 	return graph, nil
@@ -266,6 +303,17 @@ func (obj *StmtRes) Output() (*interfaces.Output, error) {
 
 	// FIXME: we could probably simplify this code...
 	for _, x := range obj.Fields {
+		if x.Condition != nil {
+			b, err := x.Condition.Value()
+			if err != nil {
+				return nil, err
+			}
+
+			if !b.Bool() { // if value exists, and is false, skip it
+				continue
+			}
+		}
+
 		typ, err := x.Value.Type()
 		if err != nil {
 			return nil, errwrap.Wrapf(err, "resource field `%s` did not return a type", x.Field)
@@ -370,8 +418,9 @@ func (obj *StmtRes) Output() (*interfaces.Output, error) {
 // StmtResField represents a single field in the parsed resource representation.
 // This does not satisfy the Stmt interface.
 type StmtResField struct {
-	Field string
-	Value interfaces.Expr
+	Field     string
+	Value     interfaces.Expr
+	Condition interfaces.Expr // the value will be used if nil or true
 }
 
 // StmtEdge is a representation of a dependency. It also supports send/recv.