From 6e8b5afd3b8ef41893ed9cc649a715463385ea26 Mon Sep 17 00:00:00 2001
From: Dustin Swan <dustin@dustinswan.com>
Date: Sun, 1 Feb 2026 01:01:48 -0700
Subject: [PATCH] parsing lambdas

---
 src/ast.ts    |  4 +++
 src/main.ts   |  2 +-
 src/parser.ts | 89 ++++++++++++++++++++++-----------------------------
 3 files changed, 43 insertions(+), 52 deletions(-)

diff --git a/src/ast.ts b/src/ast.ts
index 376c587..8edfa59 100644
--- a/src/ast.ts
+++ b/src/ast.ts
@@ -154,6 +154,10 @@ export function prettyPrint(ast: AST, indent = 0): string {
                 .join(', ');
             return `${i}{${fields}}`;
 
+        case 'lambda':
+            const params = ast.params.join(', ')
+            return `${i}(${params}) => ${prettyPrint(ast.body)}`
+
         default:
             return `${i}${ast.kind}`
     }
diff --git a/src/main.ts b/src/main.ts
index 91f2902..a0c63de 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -25,7 +25,7 @@ console.log(tokenize(str));
 // const tokens2 = tokenize("let x = (y) => 5 + y in x(3)");
 // const tokens2 = tokenize("let x = 5 in x * 4");
 // const tokens2 = tokenize("(x, y) => x + y");
-const tokens2 = tokenize('x = (y = 1; y + 1); x * 2');
+const tokens2 = tokenize(`a b \\ a + b`);
 const p2 = new Parser(tokens2);
 const ast3 = p2.parse();
 console.log(ast3);
diff --git a/src/parser.ts b/src/parser.ts
index dd843c1..359a0c7 100644
--- a/src/parser.ts
+++ b/src/parser.ts
@@ -68,11 +68,35 @@ export class Parser {
             kind === 'open-brace';
     }
 
+    private isLambdaStart(): boolean {
+        const kind = this.current().kind;
+
+        if (kind === 'backslash') {
+            return true;
+        }
+
+        if (kind !== 'ident') {
+            return false;
+        }
+
+        let offset = 1;
+        while (true) {
+            const token = this.peek(offset);
+            if (token.kind === 'backslash') return true;
+            if (token.kind !== 'ident') return false;
+            offset++;
+        }
+    }
+
     parse(): AST {
         return this.parseExpression();
     }
 
     private parseExpression(): AST {
+        if (this.isLambdaStart()) {
+            return this.parseLambda();
+        }
+
         if (this.current().kind === 'ident' && this.peek().kind === 'equals') {
             return this.parseLet();
         }
@@ -80,6 +104,20 @@ export class Parser {
         return this.parseInfix();
     }
 
+    private parseLambda(): AST {
+        const params: string[] = [];
+
+        while (this.current().kind === 'ident') {
+            const param = this.advance();
+            params.push((param as { value: string }).value);
+        }
+
+        this.expect('backslash');
+        const body = this.parseExpression();
+
+        return { kind: 'lambda', params, body };
+    }
+
     private parseLet(): AST {
         const nameToken = this.expect('ident');
         const name = (nameToken as { value: string }).value;
@@ -198,55 +236,4 @@ export class Parser {
 
         throw new Error(`Unexpected token: ${token.kind}`);
     }
-
-    /*
-    private parseLambda(): AST {
-        this.expect('open-paren');
-
-        const params: string[] = [];
-
-        if (this.current().kind !== 'close-paren') {
-            const first = this.expect('ident');
-            params.push((first as { value: string }).value);
-
-            while (this.current().kind === 'comma') {
-                this.advance();
-                const param = this.expect('ident');
-                params.push((param as { value: string }).value);
-            }
-        }
-
-        this.expect('close-paren');
-        this.expect('arrow');
-
-        const body = this.parseExpression();
-
-        return { kind: 'lambda', params, body }
-    }
-
-    private parsePostfix(): AST {
-        let expr = this.parsePrimary();
-
-        while (this.current().kind === 'open-paren') {
-            this.advance();
-
-            const args: AST[] = [];
-            if (this.current().kind !== 'close-paren') {
-                args.push(this.parseExpression());
-
-                while (this.current().kind === 'comma') {
-                    this.advance();
-                    args.push(this.parseExpression());
-                }
-            }
-
-            this.expect('close-paren');
-
-            expr = { kind: 'apply', func: expr, args };
-        }
-
-        return expr;
-    }
-    */
-
 }