Add runtime:elvish-path

Resolves #1423

0.19.0-release-notes.md

@@ -35,3 +35,5 @@ and compiled, even if it is not executed:
 
 -   A new type of interactive abbreviation: `edit:command-abbr`
     ([#1472](https://b.elv.sh/1472)).
+
+-   A new `runtime:elvish-path` variable ([#1423](https://b.elv.sh/1423)).

pkg/mods/runtime/runtime.go

@@ -2,6 +2,8 @@
 package runtime
 
 import (
+	"os"
+
 	"src.elv.sh/pkg/eval"
 	"src.elv.sh/pkg/eval/vals"
 	"src.elv.sh/pkg/eval/vars"
@@ -38,6 +40,24 @@ import (
 //
 // @cf $runtime:rc-path
 
+//elvdoc:var elvish-path
+//
+// The path to the elvish binary. If that could not be determined the value will be the empty
+// string.
+//
+// This is read-only.
+
+func elvishPath() any {
+	// TODO: Decide what to do if os.Executable returns an error. It appears that all platforms
+	// return an empty string. Which makes sense and is probably good enough for our purposes.
+	// Nonetheless, we explicitly encode that logic rather than rely on the behavior of
+	// os.Executable. TBD is whether we should instead raise an exception.
+	if binPath, err := os.Executable(); err == nil {
+		return binPath
+	}
+	return ""
+}
+
 // Ns returns the namespace for the runtime: module.
 //
 // All the public properties of the Evaler should be set before this function is
@@ -45,6 +65,7 @@ import (
 func Ns(ev *eval.Evaler) *eval.Ns {
 	return eval.BuildNsNamed("runtime").
 		AddVars(map[string]vars.Var{
+			"elvish-path":       vars.FromGet(elvishPath),
 			"lib-dirs":          vars.NewReadOnly(vals.MakeListSlice(ev.LibDirs)),
 			"rc-path":           vars.NewReadOnly(nonEmptyOrNil(ev.RcPath)),
 			"effective-rc-path": vars.NewReadOnly(nonEmptyOrNil(ev.EffectiveRcPath)),

pkg/mods/runtime/runtime_test.go

@@ -1,6 +1,7 @@
 package runtime
 
 import (
+	"os"
 	"testing"
 
 	"src.elv.sh/pkg/eval"
@@ -18,10 +19,14 @@ func TestRuntime(t *testing.T) {
 
 		ev.ExtendGlobal(eval.BuildNs().AddNs("runtime", Ns(ev)))
 	}
+
+	elvishPath, _ := os.Executable()
+
 	evaltest.TestWithSetup(t, setup,
 		That("put $runtime:lib-dirs").Puts(vals.MakeList("/lib/1", "/lib/2")),
 		That("put $runtime:rc-path").Puts("/path/to/rc.elv"),
 		That("put $runtime:effective-rc-path").Puts("/path/to/effective/rc.elv"),
+		That(`put $runtime:elvish-path`).Puts(elvishPath),
 	)
 }