lang/funcs: add "all" function

# What

This commit adds an `all` function to Terraform configuration. A
reason we might want this function is because it will enable more
powerful custom variable validations. For example:

```hcl
variable "amis" {
  type = list(object({
    id = string
  }))

  validation {
    condition = (all([
      for a in var.amis : length(a.id) > 4 && substr(a.id, 0, 4) == "ami-"
    ]))
    error_message = "The ID of at least one AMI was invalid."
  }
}
```

If this is not the intended use of custom variable validation, the `all`
function may still prove useful in other contexts where we, for example,
create a resource conditionally.

lang/funcs/collection.go

@@ -56,6 +56,41 @@ var LengthFunc = function.New(&function.Spec{
 	},
 })
 
+// AllFunc constructs a function that returns true if all elements of the
+// collection are true or "true". If the collection is empty, return true.
+var AllFunc = function.New(&function.Spec{
+	Params: []function.Parameter{
+		{
+			Name: "collection",
+			Type: cty.DynamicPseudoType,
+		},
+	},
+	Type: function.StaticReturnType(cty.Bool),
+	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
+		ty := args[0].Type()
+		if !ty.IsListType() && !ty.IsTupleType() && !ty.IsSetType() {
+			return cty.NilVal, errors.New("argument must be list, tuple, or set")
+		}
+
+		tobool := MakeToFunc(cty.Bool)
+		for it := args[0].ElementIterator(); it.Next(); {
+			_, v := it.Element()
+			got, err := tobool.Call([]cty.Value{v})
+			if err != nil {
+				return cty.False, nil
+			}
+			eq, err := stdlib.Equal(got, cty.True)
+			if err != nil {
+				return cty.NilVal, err
+			}
+			if eq.False() {
+				return cty.False, nil
+			}
+		}
+		return cty.True, nil
+	},
+})
+
 // CoalesceFunc constructs a function that takes any number of arguments and
 // returns the first one that isn't empty. This function was copied from go-cty
 // stdlib and modified so that it returns the first *non-empty* non-null element
@@ -582,6 +617,12 @@ func Length(collection cty.Value) (cty.Value, error) {
 	return LengthFunc.Call([]cty.Value{collection})
 }
 
+// All returns true if all elements of the collection are true or "true".
+// If the collection is empty, return true.
+func All(collection cty.Value) (cty.Value, error) {
+	return AllFunc.Call([]cty.Value{collection})
+}
+
 // Coalesce takes any number of arguments and returns the first one that isn't empty.
 func Coalesce(args ...cty.Value) (cty.Value, error) {
 	return CoalesceFunc.Call(args)

lang/funcs/collection_test.go

@@ -139,6 +139,99 @@ func TestLength(t *testing.T) {
 	}
 }
 
+func TestAll(t *testing.T) {
+	tests := []struct {
+		Collection cty.Value
+		Want       cty.Value
+		Err        bool
+	}{
+		{
+			cty.ListValEmpty(cty.String),
+			cty.True,
+			false,
+		},
+		{
+			cty.TupleVal([]cty.Value{}),
+			cty.True,
+			false,
+		},
+		{
+			cty.SetValEmpty(cty.Bool),
+			cty.True,
+			false,
+		},
+		{
+			cty.ListVal([]cty.Value{cty.True}),
+			cty.True,
+			false,
+		},
+		{
+			cty.ListVal([]cty.Value{cty.StringVal("true")}),
+			cty.True,
+			false,
+		},
+		{
+			cty.ListVal([]cty.Value{cty.False}),
+			cty.False,
+			false,
+		},
+		{
+			cty.ListVal([]cty.Value{cty.True, cty.False}),
+			cty.False,
+			false,
+		},
+		{
+			cty.ListVal([]cty.Value{cty.False, cty.True}),
+			cty.False,
+			false,
+		},
+		{
+			cty.ListVal([]cty.Value{cty.NumberIntVal(1)}),
+			cty.False,
+			false,
+		},
+		{
+			cty.StringVal("true"),
+			cty.False,
+			true,
+		},
+		// {
+		// 	cty.ListVal([]cty.Value{cty.ListValEmpty(cty.String)}),
+		// 	cty.False,
+		// 	false,
+		// },
+		// {
+		// 	cty.ListVal([]cty.Value{cty.True, cty.StringVal("true")}),
+		// 	cty.True,
+		// 	false,
+		// },
+		// {
+		// 	cty.ListVal([]cty.Value{cty.UnknownVal(cty.String)}),
+		// 	cty.False,
+		// 	false,
+		// },
+	}
+
+	for _, test := range tests {
+		t.Run(fmt.Sprintf("all(%#v)", test.Collection), func(t *testing.T) {
+			got, err := All(test.Collection)
+
+			if test.Err {
+				if err == nil {
+					t.Fatal("succeeded; want error")
+				}
+				return
+			} else if err != nil {
+				t.Fatalf("unexpected error: %s", err)
+			}
+
+			if !got.RawEquals(test.Want) {
+				t.Errorf("wrong result\ngot:  %#v\nwant: %#v", got, test.Want)
+			}
+		})
+	}
+}
+
 func TestCoalesce(t *testing.T) {
 	tests := []struct {
 		Values []cty.Value

lang/functions.go

@@ -33,6 +33,7 @@ func (s *Scope) Functions() map[string]function.Function {
 		s.funcs = map[string]function.Function{
 			"abs":              stdlib.AbsoluteFunc,
 			"abspath":          funcs.AbsPathFunc,
+			"all":              funcs.AllFunc,
 			"basename":         funcs.BasenameFunc,
 			"base64decode":     funcs.Base64DecodeFunc,
 			"base64encode":     funcs.Base64EncodeFunc,

lang/functions_test.go

@@ -67,6 +67,13 @@ func TestFunctions(t *testing.T) {
 			},
 		},
 
+		"all": {
+			{
+				`all([true])`,
+				cty.True,
+			},
+		},
+
 		"base64decode": {
 			{
 				`base64decode("YWJjMTIzIT8kKiYoKSctPUB+")`,

website/docs/configuration/functions/all.html.md

@@ -0,0 +1,30 @@
+---
+layout: functions
+page_title: all - Functions - Configuration Language
+sidebar_current: docs-funcs-collection-all
+description: |-
+  The all function determines whether all elements of a collection are true or
+  "true". If the collection is empty, it returns true.
+---
+
+# `all` Function
+
+-> **Note:** This page is about Terraform 0.12 and later. For Terraform 0.11 and
+earlier, see
+[0.11 Configuration Language: Interpolation Syntax](../../configuration-0-11/interpolation.html).
+
+`all` returns `true` if all elements in a given collection are `true` or
+`"true"`. It also returns `true` if the collection is empty.
+
+```hcl
+all(list)
+```
+
+## Examples
+
+```command
+> all(["true", true])
+true
+> all([true, false])
+false
+```

website/layouts/functions.erb

@@ -138,6 +138,10 @@
             <a href="#docs-funcs-collection">Collection Functions</a>
             <ul class="nav">
 
+              <li>
+                <a href="/docs/configuration/functions/all.html">all</a>
+              </li>
+
               <li>
                 <a href="/docs/configuration/functions/chunklist.html">chunklist</a>
               </li>