Bas-style interpolation in strings, closes #169

Now you can embed variables inside strings by prefixing them with
`$`. Literal `$` simply need a `\`. This aligns the string interpolation
syntax with commands, and makes it easier to switch between running
a command (`` `$some $command` ``) and echoing it (`echo("$some $command")`).

README.md

@@ -83,7 +83,7 @@ if !res.ok {
 ip = res.json().ip
 total = ip.split(".").map(int).sum()
 if total > 100 {
-    echo("The sum of [%s] is a large number, %s.", ip, total)
+    echo("The sum of [$ip] is a large number, $total.")
 }
 ```
 

docs/README.md

@@ -80,7 +80,7 @@ if !res.ok {
 ip = res.json().ip
 total = ip.split(".").map(int).sum()
 if total > 100 {
-    echo("The sum of [%s] is a large number, %s.", ip, total)
+    echo("The sum of [$ip] is a large number, $total.")
 }
 ```
 

docs/types/string.md

@@ -83,6 +83,25 @@ To test for the existence of substrings within strings use the `in` operator:
 "xyz" in "string"   # false
 ```
 
+## Interpolation
+
+You can also replace parts of the string with variables
+declared within your program using the `$` symbol:
+
+``` bash
+file = "/etc/hosts"
+x = "File name is: $file"
+echo(x) # "File name is: /etc/hosts"
+```
+
+If you need `$` literals in your command, you
+simply need to escape them with a `\`:
+
+``` bash
+"$non_existing_var" # "" since the ABS variable 'non_existing_var' doesn't exist
+"\$non_existing_var" # "$non_existing_var"
+```
+
 ## Special characters embedded in strings
 
 Double and single quoted strings behave differently if the string contains

evaluator/evaluator.go

@@ -93,7 +93,7 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 		return NULL
 
 	case *ast.StringLiteral:
-		return &object.String{Token: node.Token, Value: node.Value}
+		return &object.String{Token: node.Token, Value: util.InterpolateStringVars(node.Value, env)}
 
 	case *ast.Boolean:
 		return nativeBoolToBooleanObject(node.Value)

evaluator/evaluator_test.go

@@ -365,6 +365,22 @@ func TestStringWriters(t *testing.T) {
 	}
 }
 
+func TestStringInterpolation(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected string
+	}{
+		{`a = "123"; "abc$a"`, "abc123"},
+		{`a = "123"; "abc\$a"`, "abc$a"},
+		{`a = "123"; "$$a$$a$$a"`, "$123$123$123"},
+	}
+
+	for _, tt := range tests {
+		evaluated := testEval(tt.input)
+		testStringObject(t, evaluated, tt.expected)
+	}
+}
+
 func TestForInExpressions(t *testing.T) {
 	tests := []struct {
 		input    string

examples/ip-sum.abs

@@ -8,5 +8,5 @@ if !res.ok {
 ip = res.json().ip
 total = ip.split(".").map(int).sum()
 if total > 100 {
-    echo("The sum of [%s] is a large number, %s.", ip, total)
+    echo("The sum of [$ip] is a large number, $total.")
 }

scripts/release.abs

@@ -54,7 +54,7 @@ if !rm.ok {
 
 version = `cat ./main.go | grep "var Version"`
 version = version.slice(15, len(version) - 1)
-echo("Running builds for version %s, confirm by typing \"y\"".fmt(version))
+echo("Running builds for version $version, confirm by typing \"y\"")
 selection = stdin()
 
 if selection != "y" {
@@ -63,7 +63,7 @@ if selection != "y" {
 
 for platform in platforms {
     goos, goarch = platform.split("/")
-    output_name = "builds/abs-%s-%s-%s".fmt(version, goos, goarch)
+    output_name = "builds/abs-$version-$goos-$goarch"
     entry_point = "main.go"
 
     if goos == "windows" {