Make init value of $f~ actually identical to $nop~.

This fixes #1519.

pkg/eval/builtin_fn_misc.go

@@ -27,7 +27,6 @@ var (
 
 func init() {
 	addBuiltinFns(map[string]any{
-		"nop":        nop,
 		"kind-of":    kindOf,
 		"constantly": constantly,
 
@@ -69,6 +68,8 @@ func init() {
 // Etymology: Various languages, in particular NOP in
 // [assembly languages](https://en.wikipedia.org/wiki/NOP).
 
+var nopGoFn = NewGoFn("nop", nop)
+
 func nop(opts RawOptions, args ...any) {
 	// Do nothing
 }

pkg/eval/builtin_ns.go

@@ -104,15 +104,16 @@ import (
 // @cf buildinfo
 
 var builtinNs = BuildNsNamed("").AddVars(map[string]vars.Var{
-	"_":         vars.NewBlackhole(),
-	"pid":       vars.NewReadOnly(strconv.Itoa(syscall.Getpid())),
-	"ok":        vars.NewReadOnly(OK),
-	"nil":       vars.NewReadOnly(nil),
-	"true":      vars.NewReadOnly(true),
-	"false":     vars.NewReadOnly(false),
-	"buildinfo": vars.NewReadOnly(buildinfo.Value),
-	"version":   vars.NewReadOnly(buildinfo.Value.Version),
-	"paths":     vars.NewEnvListVar("PATH"),
+	"_":              vars.NewBlackhole(),
+	"pid":            vars.NewReadOnly(strconv.Itoa(syscall.Getpid())),
+	"ok":             vars.NewReadOnly(OK),
+	"nil":            vars.NewReadOnly(nil),
+	"true":           vars.NewReadOnly(true),
+	"false":          vars.NewReadOnly(false),
+	"buildinfo":      vars.NewReadOnly(buildinfo.Value),
+	"version":        vars.NewReadOnly(buildinfo.Value.Version),
+	"paths":          vars.NewEnvListVar("PATH"),
+	"nop" + FnSuffix: vars.NewReadOnly(nopGoFn),
 })
 
 func addBuiltinFns(fns map[string]any) {

pkg/eval/builtin_special_test.go

@@ -41,6 +41,7 @@ func TestVar(t *testing.T) {
 		// Declaring a variable whose name ends in "~" initializes it to the
 		// builtin nop function.
 		That("var cmd~; cmd &ignored-opt ignored-arg").DoesNothing(),
+		That("var cmd~; is $cmd~ $nop~").Puts(true),
 		// Declaring multiple variables
 		That("var x y", "put $x $y").Puts(nil, nil),
 		// Declaring one variable with initial value

pkg/eval/closure.go

@@ -148,20 +148,15 @@ func (c *Closure) Call(fm *Frame, args []any, opts map[string]any) error {
 	return exc
 }
 
-var (
-	fnDefault = NewGoFn("nop~", nop)
-	nsDefault = &Ns{}
-)
-
 // MakeVarFromName creates a Var with a suitable type constraint inferred from
 // the name.
 func MakeVarFromName(name string) vars.Var {
 	switch {
 	case strings.HasSuffix(name, FnSuffix):
-		val := fnDefault
+		val := nopGoFn
 		return vars.FromPtr(&val)
 	case strings.HasSuffix(name, NsSuffix):
-		val := nsDefault
+		val := &Ns{}
 		return vars.FromPtr(&val)
 	default:
 		return vars.FromInit(nil)