diff --git a/lang/types/type.go b/lang/types/type.go
index e8089236..c29ab6e9 100644
--- a/lang/types/type.go
+++ b/lang/types/type.go
@@ -33,15 +33,20 @@ import (
 	"fmt"
 	"net"
 	"reflect"
+	"strconv"
 	"strings"
 
 	"github.com/purpleidea/mgmt/util"
+	"github.com/purpleidea/mgmt/util/disjoint"
 	"github.com/purpleidea/mgmt/util/errwrap"
 )
 
 const (
 	// StructTag is the key we use in struct field names for key mapping.
 	StructTag = "lang"
+
+	// MaxInt8 is 127. It's max uint8: ^uint8(0), then we >> 1 for max int8.
+	MaxInt8 = int((^uint8(0)) >> 1)
 )
 
 // Basic types defined here as a convenience for use with Type.Cmp(X).
@@ -71,6 +76,8 @@ const (
 	KindStruct
 	KindFunc
 	KindVariant
+
+	KindUnification = Kind(MaxInt8) // keep this last
 )
 
 // Type is the datastructure representing any type. It can be recursive for
@@ -85,6 +92,20 @@ type Type struct {
 	Ord []string
 	Out *Type // if Kind == Func, use Map and Ord for Input, Out for Output
 	Var *Type // if Kind == Variant, use Var only
+
+	// unification variable (question mark, eg ?1, ?2)
+	Uni *Elem // if Kind == Unification (optional) use Uni only
+}
+
+// Elem is the type used for the unification variable in the Uni field of Type.
+// We create this alias here to avoid needing to write *disjoint.Elem[*Type] all
+// over. This is a golang type alias. These should be created with NewElem.
+type Elem = disjoint.Elem[*Type]
+
+// NewElem creates a new set with one element and returns the sole element (the
+// representative element) of that set.
+func NewElem() *Elem {
+	return disjoint.NewElem[*Type]()
 }
 
 // TypeOf takes a reflect.Type and returns an equivalent *Type. It removes any
@@ -340,6 +361,14 @@ func ConfigurableTypeOf(t reflect.Type, opts ...TypeOfOption) (*Type, error) {
 
 // NewType creates the Type from the string representation.
 func NewType(s string) *Type {
+	table := make(map[uint]*Elem)
+	return newType(s, table)
+}
+
+// newType creates the Type from the string representation. This private version
+// takes a table so that we can collect unification variables as we see them and
+// return a type with correctly unified unification variables.
+func newType(s string, table map[uint]*Elem) *Type {
 	switch s {
 	case "bool":
 		return &Type{
@@ -361,7 +390,7 @@ func NewType(s string) *Type {
 
 	// KindList
 	if strings.HasPrefix(s, "[]") {
-		val := NewType(s[len("[]"):])
+		val := newType(s[len("[]"):], table)
 		if val == nil {
 			return nil
 		}
@@ -395,11 +424,11 @@ func NewType(s string) *Type {
 			return nil
 		}
 
-		key := NewType(strings.Trim(s[:found], " "))
+		key := newType(strings.Trim(s[:found], " "), table)
 		if key == nil {
 			return nil
 		}
-		val := NewType(strings.Trim(s[found+1:], " "))
+		val := newType(strings.Trim(s[found+1:], " "), table)
 		if val == nil {
 			return nil
 		}
@@ -457,7 +486,7 @@ func NewType(s string) *Type {
 				trim = 1
 			}
 
-			typ := NewType(strings.Trim(s[:found+1-trim], " "))
+			typ := newType(strings.Trim(s[:found+1-trim], " "), table)
 			if typ == nil {
 				return nil
 			}
@@ -557,7 +586,7 @@ func NewType(s string) *Type {
 				trim = 1
 			}
 
-			typ := NewType(strings.Trim(s[:found+1-trim], " "))
+			typ := newType(strings.Trim(s[:found+1-trim], " "), table)
 			if typ == nil {
 				return nil
 			}
@@ -568,7 +597,7 @@ func NewType(s string) *Type {
 		// return type
 		var tail *Type
 		if out != "" { // allow functions with no return type (in parser)
-			tail = NewType(out)
+			tail = newType(out, table)
 			if tail == nil {
 				return nil
 			}
@@ -589,6 +618,47 @@ func NewType(s string) *Type {
 		}
 	}
 
+	// KindUnification
+	if strings.HasPrefix(s, "?") {
+		// find end of number...
+		var length = 0 // number of digits
+		for i := len("?"); i < len(s); i++ {
+			c := s[i]
+			if length == 0 && c == '0' {
+				return nil // can't start with a zero
+			}
+
+			// Check manually because strconv.ParseUint accepts ^0x.
+			if '0' <= c && c <= '9' {
+				length++
+				continue
+			}
+			return nil // invalid char
+		}
+
+		v := s[len("?") : len("?")+length]
+		n, err := strconv.ParseUint(v, 10, 32) // base 10, 32 bits
+		if err != nil {
+			return nil // programming error or overflow
+		}
+		num := uint(n)
+
+		// XXX: Should we instead always return new unification
+		// variables, but call .Union() on all of the ones that have the
+		// same integer? Sam says they are equivalent.
+		uni, exists := table[num]
+		if !exists {
+			uni = NewElem()  // unification variable, eg: ?1
+			table[num] = uni // store
+		}
+
+		// return a new type, may have an existing unification variable
+		return &Type{
+			Kind: KindUnification,
+			Uni:  uni, // unification variable, eg: ?1
+		}
+	}
+
 	return nil // error (this also matches the empty string as input)
 }
 
@@ -617,12 +687,21 @@ func (obj *Type) New() Value {
 		return NewFunc(obj)
 	case KindVariant:
 		return NewVariant(obj)
+	case KindUnification:
+		panic("can't make new value from unification variable kind")
 	}
 	panic("malformed type")
 }
 
 // String returns the textual representation for this type.
 func (obj *Type) String() string {
+	table := make(map[*Elem]uint)
+	return obj.string(table)
+}
+
+// string returns the textual representation for this type. This is a private
+// helper function that is used by the real String function.
+func (obj *Type) string(table map[*Elem]uint) string {
 	switch obj.Kind {
 	case KindBool:
 		return "bool"
@@ -637,13 +716,13 @@ func (obj *Type) String() string {
 		if obj.Val == nil {
 			panic("malformed list type")
 		}
-		return "[]" + obj.Val.String()
+		return "[]" + obj.Val.string(table)
 
 	case KindMap:
 		if obj.Key == nil || obj.Val == nil {
 			panic("malformed map type")
 		}
-		return fmt.Sprintf("map{%s: %s}", obj.Key.String(), obj.Val.String())
+		return fmt.Sprintf("map{%s: %s}", obj.Key.string(table), obj.Val.string(table))
 
 	case KindStruct: // {a bool; b int}
 		if obj.Map == nil {
@@ -661,7 +740,7 @@ func (obj *Type) String() string {
 			if t == nil {
 				panic("malformed struct field")
 			}
-			s[i] = fmt.Sprintf("%s %s", k, t.String())
+			s[i] = fmt.Sprintf("%s %s", k, t.string(table))
 		}
 		return fmt.Sprintf("struct{%s}", strings.Join(s, "; "))
 
@@ -684,17 +763,37 @@ func (obj *Type) String() string {
 
 			// We need to print function arg names for Copy() to use
 			// the String() hack here and avoid erasing them here!
-			//s[i] = t.String()
-			s[i] = fmt.Sprintf("%s %s", k, t.String()) // strict
+			//s[i] = t.string(table)
+			s[i] = fmt.Sprintf("%s %s", k, t.string(table)) // strict
 		}
 		var out string
 		if obj.Out != nil {
-			out = fmt.Sprintf(" %s", obj.Out.String())
+			out = fmt.Sprintf(" %s", obj.Out.string(table))
 		}
 		return fmt.Sprintf("func(%s)%s", strings.Join(s, ", "), out)
 
 	case KindVariant:
 		return "variant"
+
+	case KindUnification:
+		if obj.Uni == nil {
+			panic("malformed unification variable")
+		}
+
+		// XXX: Should we instead run .IsConnected() on the two Elem
+		// unification variables to determine if they should have the
+		// same integer representation when printing them?
+		num, exists := table[obj.Uni]
+		if !exists {
+			for _, n := range table {
+				num = max(num, n)
+			}
+			num++                // add 1
+			table[obj.Uni] = num // store
+		}
+
+		//fmt.Printf("?%d: %p\n", int(num), obj.Uni.Find()) // debug
+		return "?" + strconv.Itoa(int(num))
 	}
 
 	panic("malformed type")
@@ -702,6 +801,14 @@ func (obj *Type) String() string {
 
 // Cmp compares this type to another.
 func (obj *Type) Cmp(typ *Type) error {
+	table1 := make(map[*Elem]uint) // for obj
+	table2 := make(map[*Elem]uint) // for typ
+	return obj.cmp(typ, table1, table2)
+}
+
+// cmp compares this type to another. This is a private helper function that is
+// used by the real Cmp function.
+func (obj *Type) cmp(typ *Type, table1, table2 map[*Elem]uint) error {
 	if obj == nil || typ == nil {
 		return fmt.Errorf("cannot compare to nil")
 	}
@@ -709,10 +816,10 @@ func (obj *Type) Cmp(typ *Type) error {
 	// TODO: is this correct?
 	// recurse into variants if we want base type comparisons
 	//if obj.Kind == KindVariant {
-	//	return obj.Var.Cmp(t)
+	//	return obj.Var.cmp(t, table1, table2)
 	//}
 	//if t.Kind == KindVariant {
-	//	return obj.Cmp(t.Var)
+	//	return obj.cmp(t.Var, table1, table2)
 	//}
 
 	if obj.Kind != typ.Kind {
@@ -732,14 +839,14 @@ func (obj *Type) Cmp(typ *Type) error {
 		if obj.Val == nil || typ.Val == nil {
 			panic("malformed list type")
 		}
-		return obj.Val.Cmp(typ.Val)
+		return obj.Val.cmp(typ.Val, table1, table2)
 
 	case KindMap:
 		if obj.Key == nil || obj.Val == nil || typ.Key == nil || typ.Val == nil {
 			panic("malformed map type")
 		}
-		kerr := obj.Key.Cmp(typ.Key)
-		verr := obj.Val.Cmp(typ.Val)
+		kerr := obj.Key.cmp(typ.Key, table1, table2)
+		verr := obj.Val.cmp(typ.Val, table1, table2)
 		if kerr != nil && verr != nil {
 			return errwrap.Append(kerr, verr) // two errors
 		}
@@ -775,7 +882,7 @@ func (obj *Type) Cmp(typ *Type) error {
 			if t1 == nil || t2 == nil {
 				panic("malformed struct field")
 			}
-			if err := t1.Cmp(t2); err != nil {
+			if err := t1.cmp(t2, table1, table2); err != nil {
 				return err
 			}
 		}
@@ -806,7 +913,7 @@ func (obj *Type) Cmp(typ *Type) error {
 		//	if t1 == nil || t2 == nil {
 		//		panic("malformed func arg")
 		//	}
-		//	if err := t1.Cmp(t2); err != nil {
+		//	if err := t1.cmp(t2, table1, table2); err != nil {
 		//		return err
 		//	}
 		//}
@@ -829,13 +936,13 @@ func (obj *Type) Cmp(typ *Type) error {
 				panic("malformed func arg")
 			}
 
-			if err := t1.Cmp(t2); err != nil {
+			if err := t1.cmp(t2, table1, table2); err != nil {
 				return err
 			}
 		}
 
 		if obj.Out != nil || typ.Out != nil {
-			if err := obj.Out.Cmp(typ.Out); err != nil {
+			if err := obj.Out.cmp(typ.Out, table1, table2); err != nil {
 				return err
 			}
 		}
@@ -848,6 +955,39 @@ func (obj *Type) Cmp(typ *Type) error {
 		}
 		// TODO: should we Cmp obj.Var with typ.Var ? -- not necessarily
 		return nil
+
+	// used for testing
+	case KindUnification:
+		if obj.Uni == nil || typ.Uni == nil {
+			panic("malformed unification variable")
+		}
+
+		// If both types store and lookup variables symmetrically and in
+		// the same order, then the count's should also match.
+		// XXX: Should we instead run .IsConnected() on the two Elem
+		// unification variables to determine if they should have the
+		// same integer representation when printing them?
+		num1, exists := table1[obj.Uni]
+		if !exists {
+			for _, n := range table1 {
+				num1 = max(num1, n)
+			}
+			num1++                 // add 1
+			table1[obj.Uni] = num1 // store
+		}
+
+		num2, exists := table2[typ.Uni]
+		if !exists {
+			for _, n := range table2 {
+				num2 = max(num2, n)
+			}
+			num2++                 // add 1
+			table2[typ.Uni] = num2 // store
+		}
+		if num1 != num2 {
+			return fmt.Errorf("unbalanced unification variables")
+		}
+		return nil
 	}
 	return fmt.Errorf("unknown kind")
 }
@@ -1040,6 +1180,97 @@ func (obj *Type) HasVariant() bool {
 
 	case KindVariant:
 		return true // found it!
+
+	case KindUnification:
+		return false // TODO: Do we want to panic here instead?
+	}
+
+	panic("malformed type")
+}
+
+// HasUni tells us if the type contains any unification variables.
+func (obj *Type) HasUni() bool {
+	if obj == nil {
+		return false
+	}
+	if obj.Uni != nil {
+		return true // found it (by this method)
+	}
+
+	switch obj.Kind {
+	case KindBool:
+		return false
+	case KindStr:
+		return false
+	case KindInt:
+		return false
+	case KindFloat:
+		return false
+
+	case KindList:
+		if obj.Val == nil {
+			panic("malformed list type")
+		}
+		return obj.Val.HasUni()
+
+	case KindMap:
+		if obj.Key == nil || obj.Val == nil {
+			panic("malformed map type")
+		}
+		return obj.Key.HasUni() || obj.Val.HasUni()
+
+	case KindStruct: // {a bool; b int}
+		if obj.Map == nil {
+			panic("malformed struct type")
+		}
+		if len(obj.Map) != len(obj.Ord) {
+			panic("malformed struct length")
+		}
+		for _, k := range obj.Ord {
+			t, ok := obj.Map[k]
+			if !ok {
+				panic("malformed struct order")
+			}
+			if t == nil {
+				panic("malformed struct field")
+			}
+			if t.HasUni() {
+				return true
+			}
+		}
+		return false
+
+	case KindFunc:
+		if obj.Map == nil {
+			panic("malformed func type")
+		}
+		if len(obj.Map) != len(obj.Ord) {
+			panic("malformed func length")
+		}
+		for _, k := range obj.Ord {
+			t, ok := obj.Map[k]
+			if !ok {
+				panic("malformed func order")
+			}
+			if t == nil {
+				panic("malformed func field")
+			}
+			if t.HasUni() {
+				return true
+			}
+		}
+		if obj.Out != nil {
+			if obj.Out.HasUni() {
+				return true
+			}
+		}
+		return false
+
+	case KindVariant:
+		return obj.Var.HasUni()
+
+	case KindUnification:
+		return true // found it!
 	}
 
 	panic("malformed type")
@@ -1053,6 +1284,7 @@ func (obj *Type) HasVariant() bool {
 // string, and if it is compatible with the variant type it will be "variant"...
 // Comparing to a partial can only match "impossible" (error) or possible (nil).
 // This now also supports comparing a partial type to a variant type as well...
+// TODO: Should we support KindUnification somehow?
 func (obj *Type) ComplexCmp(typ *Type) (string, error) {
 	// match simple "placeholder" variants... skip variants w/ sub types
 	isVariant := func(t *Type) bool { return t != nil && t.Kind == KindVariant && t.Var == nil }
diff --git a/lang/types/type_test.go b/lang/types/type_test.go
index 23c169ca..b8b3c57c 100644
--- a/lang/types/type_test.go
+++ b/lang/types/type_test.go
@@ -1498,6 +1498,270 @@ func TestTypeCopy0(t *testing.T) {
 	}
 }
 
+func TestUni0(t *testing.T) {
+	// good type strings
+	if NewType("?1") == nil {
+		t.Errorf("unexpected nil type")
+	}
+	if NewType("?123") == nil {
+		t.Errorf("unexpected nil type")
+	}
+	if NewType("[]?123") == nil {
+		t.Errorf("unexpected nil type")
+	}
+	if NewType("map{?123: ?123}") == nil {
+		t.Errorf("unexpected nil type")
+	}
+
+	// bad type strings
+	if typ := NewType("?0"); typ != nil {
+		t.Errorf("expected nil type, got: %v", typ)
+	}
+	if typ := NewType("?00"); typ != nil {
+		t.Errorf("expected nil type, got: %v", typ)
+	}
+	if typ := NewType("?000000000000000000000"); typ != nil {
+		t.Errorf("expected nil type, got: %v", typ)
+	}
+}
+
+func TestUni1(t *testing.T) {
+	// functions with named types...
+	testCases := map[string]*Type{
+		// good type strings
+		"?1": {
+			Kind: KindUnification,
+			Uni:  NewElem(),
+		},
+		"?123": {
+			Kind: KindUnification,
+			Uni:  NewElem(),
+		},
+		"[]?123": {
+			Kind: KindList,
+			Val: &Type{
+				Kind: KindUnification,
+				Uni:  NewElem(),
+			},
+		},
+
+		// bad type strings
+		"?0":     nil,
+		"?00":    nil,
+		"?00000": nil,
+		"?-1":    nil,
+		"?-42":   nil,
+		"?0x42":  nil, // hexadecimal
+		"?013":   nil, // octal
+	}
+
+	for str, val := range testCases { // run all the tests
+		// for debugging
+		//if str != "?0" {
+		//continue
+		//}
+
+		// check the type
+		typ := NewType(str)
+		//t.Logf("str: %+v", str)
+		//t.Logf("typ: %+v", typ)
+
+		if val == nil { // catch error cases
+			if typ != nil {
+				t.Errorf("invalid type: `%s` did not match expected nil", str)
+			}
+			continue
+		}
+
+		if err := typ.Cmp(val); err != nil {
+			t.Errorf("type: `%s` did not match expected: `%v`", str, err)
+			return
+		}
+	}
+}
+
+func TestUniCmp0(t *testing.T) {
+	type test struct { // an individual test
+		name string
+		typ1 *Type
+		typ2 *Type
+		err  bool   // expected err ?
+		str  string // expected output str
+	}
+	testCases := []test{}
+
+	testCases = append(testCases, test{
+		name: "simple ?1 compare",
+		typ1: NewType("?1"),
+		typ2: NewType("?1"),
+		err:  false,
+	})
+	testCases = append(testCases, test{
+		name: "different ?1 compare",
+		typ1: NewType("?13"), // they don't need to be the same
+		typ2: NewType("?42"),
+		err:  false,
+	})
+	testCases = append(testCases, test{
+		name: "duplicate type unification variables",
+		// the type unification variables should be the same
+		typ1: NewType("map{?123:?123}"),
+		typ2: &Type{
+			Kind: KindMap,
+			Key: &Type{
+				Kind: KindUnification,
+				Uni:  NewElem(),
+			},
+			Val: &Type{
+				Kind: KindUnification,
+				Uni:  NewElem(),
+			},
+		},
+		err: true,
+	})
+	{
+		uni0 := NewElem()
+		testCases = append(testCases, test{
+			name: "same type unification variables in map",
+			// the type unification variables should be the same
+			typ1: NewType("map{?123:?123}"),
+			typ2: &Type{
+				Kind: KindMap,
+				Key: &Type{
+					Kind: KindUnification,
+					Uni:  uni0,
+				},
+				Val: &Type{
+					Kind: KindUnification,
+					Uni:  uni0,
+				},
+			},
+			err: false,
+		})
+	}
+	{
+		uni1 := NewElem()
+		uni2 := NewElem()
+		uni3 := NewElem()
+		// XXX: should we instead have uni0 for the return type and
+		// .Union() it with uni2 ?
+		//uni0 := NewElem()
+		//uni2.Union(uni0)
+		testCases = append(testCases, test{
+			name: "duplicate type unification variables in functions",
+			// the type unification variables should be the same
+			typ1: NewType("func(?13, ?42, ?4, int) ?42"),
+			typ2: &Type{
+				Kind: KindFunc,
+				Map: map[string]*Type{
+					"a": {
+						Kind: KindUnification,
+						Uni:  uni1,
+					},
+					"b": {
+						Kind: KindUnification,
+						Uni:  uni2,
+					},
+					"c": {
+						Kind: KindUnification,
+						Uni:  uni3,
+					},
+					"d": TypeInt,
+				},
+				Ord: []string{"a", "b", "c", "d"},
+				Out: &Type{
+					Kind: KindUnification,
+					Uni:  uni2, // same as the second arg
+				},
+			},
+			err: false,
+		})
+	}
+	{
+		uni1 := NewElem()
+		uni2 := NewElem()
+		// XXX: should we instead have uni0 for the return type and
+		// .Union() it with uni2 ?
+		//uni0 := NewElem()
+		//uni2.Union(uni0)
+		testCases = append(testCases, test{
+			name: "duplicate type unification variables in functions unbalanced",
+			// the type unification variables should be the same
+			typ1: NewType("func(?13, ?42, ?4, int) ?42"),
+			typ2: &Type{
+				Kind: KindFunc,
+				Map: map[string]*Type{
+					"a": {
+						Kind: KindUnification,
+						Uni:  uni1,
+					},
+					"b": {
+						Kind: KindUnification,
+						Uni:  uni2,
+					},
+					"c": {
+						Kind: KindUnification,
+						Uni:  uni1, // must not match!
+					},
+					"d": TypeInt,
+				},
+				Ord: []string{"a", "b", "c", "d"},
+				Out: &Type{
+					Kind: KindUnification,
+					Uni:  uni2, // same as the second arg
+				},
+			},
+			err: true,
+		})
+	}
+
+	if testing.Short() {
+		t.Logf("available tests:")
+	}
+	names := []string{}
+	for index, tc := range testCases { // run all the tests
+		if tc.name == "" {
+			t.Errorf("test #%d: not named", index)
+			continue
+		}
+		if util.StrInList(tc.name, names) {
+			t.Errorf("test #%d: duplicate sub test name of: %s", index, tc.name)
+			continue
+		}
+		names = append(names, tc.name)
+
+		testName := fmt.Sprintf("test #%d (%s)", index, tc.name)
+		if testing.Short() { // make listing tests easier
+			t.Logf("%s", testName)
+			continue
+		}
+		t.Run(testName, func(t *testing.T) {
+			typ1, typ2, err := tc.typ1, tc.typ2, tc.err
+
+			// the reverse should probably match the forward version
+			err1 := typ1.Cmp(typ2)
+			err2 := typ2.Cmp(typ1)
+
+			if err && err1 == nil {
+				t.Errorf("test #%d: FAIL", index)
+				t.Errorf("test #%d: expected error, got nil", index)
+			}
+			if !err && err1 != nil {
+				t.Errorf("test #%d: FAIL", index)
+				t.Errorf("test #%d: unexpected error: %+v", index, err1)
+			}
+			if err && err2 == nil {
+				t.Errorf("test #%d: FAIL", index)
+				t.Errorf("test #%d: expected error, got nil", index)
+			}
+			if !err && err2 != nil {
+				t.Errorf("test #%d: FAIL", index)
+				t.Errorf("test #%d: unexpected error: %+v", index, err2)
+			}
+		})
+	}
+}
+
 func TestTypeOf0(t *testing.T) {
 	// TODO: implement testing of the TypeOf function
 	// TODO: implement testing TypeOf for struct field name mappings