evaluating pattern matching

src/ast.ts

@@ -126,8 +126,18 @@ export function prettyPrint(ast: AST, indent = 0): string {
     const i = ' '.repeat(indent);
 
     switch (ast.kind) {
-        case 'literal':
-            return `${i}${ast.value}`;
+        case 'literal': {
+            const val = ast.value;
+            switch (val.kind) {
+                case 'int':
+                case 'float':
+                    return `${i}${val.value}`;
+                case 'string':
+                    return `${i}"${val.value}"`;
+                default:
+                    return `${i}${val.kind}`;
+            }
+        }
 
         case 'variable':
             return `${i}${ast.name}`;
@@ -171,7 +181,6 @@ export function prettyPrint(ast: AST, indent = 0): string {
                 .join('\n');
             return `${i}match ${expr}\n${cases}`;
 
-
         default:
             return `${i}${ast.kind}`
     }

src/interpreter.ts

@@ -1,4 +1,4 @@
-import type { AST } from './ast';
+import type { AST, Pattern } from './ast';
 import type { Env } from './env';
 import type { Value } from './types';
 
@@ -136,6 +136,24 @@ export function evaluate(ast: AST, env: Env): Value {
             return evaluate(func.body, callEnv);
         }
 
+        case 'match': {
+            const value = evaluate(ast.expr, env);
+
+            for (const matchCase of ast.cases) {
+                const bindings = matchPattern(value, matchCase.pattern);
+
+                if (bindings !== null) {
+                    const newEnv = new Map(env);
+                    for (const [name, val] of Object.entries(bindings)) {
+                        newEnv.set(name, val);
+                    }
+                    return evaluate(matchCase.result, newEnv);
+                }
+            }
+
+            throw new Error('Non-exhaustive pattern match');
+        }
+
         default:
             throw new Error('Syntax Error');
     }
@@ -206,3 +224,66 @@ function evaluateBuiltIn(op: string, left: Value, right: Value): Value {
 
     throw new Error(`Unknown built-in: ${op}`);
 }
+
+type Bindings = { [key: string]: Value };
+
+function matchPattern(value: Value, pattern: Pattern): Bindings | null {
+    switch (pattern.kind) {
+        case 'wildcard':
+            return {};
+
+        case 'var':
+            return { [pattern.name]: value };
+
+        case 'literal':
+            if (value.kind === 'int' || value.kind === 'float' || value.kind === 'string') {
+                if (value.value === pattern.value) {
+                    return {};
+                }
+            }
+            return null;
+
+        case 'constructor': {
+            if (value.kind !== 'constructor') return null;
+            if (value.name !== pattern.name) return null;
+            if (value.args.length !== pattern.args.length) return null;
+
+            const bindings: Bindings = {};
+            for (let i = 0; i < pattern.args.length; i++) {
+                const argBindings = matchPattern(value.args[i], pattern.args[i]);
+                if (argBindings === null) return null;
+                Object.assign(bindings, argBindings);
+            }
+            return bindings;
+        }
+
+        case 'list': {
+            if (value.kind !== 'list') return null;
+            if (value.elements.length !== pattern.elements.length) return null;
+
+            const bindings: Bindings = {};
+            for (let i = 0; i < pattern.elements.length; i++) {
+                const elemBindings = matchPattern(value.elements[i], pattern.elements[i]);
+                if (elemBindings === null) return null;
+                Object.assign(bindings, elemBindings);
+            }
+            return bindings;
+        }
+
+        case 'record': {
+            if (value.kind !== 'record') return null;
+
+            const bindings: Bindings = {};
+            for (const [fieldName, fieldPattern] of Object.entries(pattern.fields)) {
+                const fieldValue = value.fields[fieldName];
+                if (fieldValue === undefined) return null;
+
+                const fieldBindings = matchPattern(fieldValue, fieldPattern);
+                if (fieldBindings === null) return null;
+                Object.assign(bindings, fieldBindings);
+            }
+            return bindings;
+        }
+    }
+
+}

src/main.ts

@@ -13,7 +13,7 @@ function e(str: string) {
     console.log(ast);
     console.log(prettyPrint(ast));
     const env: Env = new Map();
-    // console.log(evaluate(ast, env));
+    console.log(evaluate(ast, env));
 }
 
 e('add1 = (x \\ x + 1); add1 3');
@@ -26,5 +26,8 @@ e('add1 = (x \\ x + 1); 3 > add1');
 e('[1, 2] & [3, 4]');
 e('"abc" & "def"');
 // e('n | 0 \\ 1 | _ \\ 99');
-e('m | Some x \\ 1 | None \\ 0');
-e('head = list \\ list | [x, _] \\ Some x | [] \\ None; head');
+// e('m | Some x \\ 1 | None \\ 0');
+// e('head = list \\ list | [x, _] \\ Some x | [] \\ None; head');
+e('n = 5; n | 5 \\ "five" | _ \\ "other"');
+e('list = [1, 2, 3]; list | [x, y, z] \\ x + y + z');
+e('point = {x = 5, y = 10}; point | {x = px, y = py} \\ px + py');

src/parser.ts

@@ -179,7 +179,7 @@ export class Parser {
         // Record
         if (token.kind === 'open-brace') {
             this.advance();
-            const fields: { [key: string]: Pattern }= [];
+            const fields: { [key: string]: Pattern } = {};
             let first = true;
 
             while (this.current().kind !== 'close-brace') {
@@ -193,7 +193,7 @@ export class Parser {
             }
 
             this.expect('close-brace');
-            return { kind: 'list', fields };
+            return { kind: 'record', fields };
         }
 
         // Parens