implement pathexpand

lang/funcs/filesystem.go

@@ -91,6 +91,22 @@ var DirnameFunc = function.New(&function.Spec{
 	},
 })
 
+// PathExpandFunc constructs a function that expands a leading ~ character to the current user's home directory.
+var PathExpandFunc = function.New(&function.Spec{
+	Params: []function.Parameter{
+		{
+			Name: "path",
+			Type: cty.String,
+		},
+	},
+	Type: function.StaticReturnType(cty.String),
+	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+
+		homePath, err := homedir.Expand(args[0].AsString())
+		return cty.StringVal(homePath), err
+	},
+})
+
 // File reads the contents of the file at the given path.
 //
 // The file must contain valid UTF-8 bytes, or this function will return an error.
@@ -134,3 +150,14 @@ func Basename(path cty.Value) (cty.Value, error) {
 func Dirname(path cty.Value) (cty.Value, error) {
 	return DirnameFunc.Call([]cty.Value{path})
 }
+
+// Pathexpand takes a string that might begin with a `~` segment, and if so it replaces that segment with
+// the current user's home directory path.
+//
+// The underlying function implementation works only with the path string and does not access the filesystem itself.
+// It is therefore unable to take into account filesystem features such as symlinks.
+//
+// If the leading segment in the path is not `~` then the given path is returned unmodified.
+func Pathexpand(path cty.Value) (cty.Value, error) {
+	return PathExpandFunc.Call([]cty.Value{path})
+}

lang/funcs/filesystem_test.go

@@ -2,8 +2,10 @@ package funcs
 
 import (
 	"fmt"
+	"path/filepath"
 	"testing"
 
+	homedir "github.com/mitchellh/go-homedir"
 	"github.com/zclconf/go-cty/cty"
 )
 
@@ -191,3 +193,58 @@ func TestDirname(t *testing.T) {
 		})
 	}
 }
+
+func TestPathExpand(t *testing.T) {
+	homePath, err := homedir.Dir()
+	if err != nil {
+		t.Fatalf("Error getting home directory: %v", err)
+	}
+
+	tests := []struct {
+		Path cty.Value
+		Want cty.Value
+		Err  bool
+	}{
+		{
+			cty.StringVal("~/test-file"),
+			cty.StringVal(filepath.Join(homePath, "test-file")),
+			false,
+		},
+		{
+			cty.StringVal("~/another/test/file"),
+			cty.StringVal(filepath.Join(homePath, "another/test/file")),
+			false,
+		},
+		{
+			cty.StringVal("/root/file"),
+			cty.StringVal("/root/file"),
+			false,
+		},
+		{
+			cty.StringVal("/"),
+			cty.StringVal("/"),
+			false,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(fmt.Sprintf("Dirname(%#v)", test.Path), func(t *testing.T) {
+			got, err := Pathexpand(test.Path)
+
+			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)
+			}
+		})
+	}
+}

lang/functions.go

@@ -73,7 +73,7 @@ func (s *Scope) Functions() map[string]function.Function {
 			"md5":          unimplFunc, // TODO
 			"merge":        unimplFunc, // TODO
 			"min":          stdlib.MinFunc,
-			"pathexpand":   unimplFunc, // TODO
+			"pathexpand":   funcs.PathExpandFunc,
 			"pow":          unimplFunc, // TODO
 			"replace":      unimplFunc, // TODO
 			"rsadecrypt":   unimplFunc, // TODO