Adding `debug` builtin. Allowing us to assign to _ in lets

src/builtins.ts

@@ -461,5 +461,16 @@ export const builtins: { [name: string]: NativeFunction } = {
             const metrics = measureCtx.measureText(str);
             return { kind: 'float', value: metrics.width };
         }
+    },
+
+    'debug': {
+        kind: 'native',
+        name: 'debug',
+        arity: 2,
+        fn: (label, value) => {
+            const l = expectString(label, 'debug');
+            console.log(`[DEBUG] ${l}: `, value);
+            return value;
+        }
     }
 }

src/interpreter.ts

@@ -87,6 +87,11 @@ export function evaluate(ast: AST, env: Env): Value {
 
             const val = evaluate(ast.value, newEnv);
 
+            // Don't bind _
+            if (ast.name !== '_') {
+                newEnv.set(ast.name, val);
+            }
+
             newEnv.set(ast.name, val);
 
             return evaluate(ast.body, newEnv);

src/parser.ts

@@ -114,7 +114,7 @@ export class Parser {
         }
 
         // Let
-        if (this.current().kind === 'ident' && this.peek().kind === 'equals') {
+        if ((this.current().kind === 'ident' || this.current().kind === 'underscore') && this.peek().kind === 'equals') {
             return this.parseLet();
         }
 
@@ -278,8 +278,20 @@ export class Parser {
     }
 
     private parseLet(): AST {
-        const nameToken = this.expect('ident');
-        const name = (nameToken as { value: string }).value;
+        const nameToken = this.current();
+        let name: string;
+
+        if (nameToken.kind === 'underscore') {
+            name = '_';
+            this.advance();
+        } else if (nameToken.kind === 'ident') {
+            name = (nameToken as { value: string }).value;
+            this.advance();
+        } else {
+            throw new Error(`Expected ident or underscore, got ${nameToken.kind}`);
+
+        }
+
         this.expect('equals');
         const value = this.parseExpressionNoMatch();
         this.expect('semicolon');

src/ui-components.cg

@@ -11,6 +11,7 @@ button = config \
     };
 
 insertChar = text pos char \
+  _ = debug "insertChar" char;
   before = slice text 0 pos;
   after = slice text pos (len text);
   before & char & after;