parsing pattern matching

src/ast.ts

@@ -127,7 +127,7 @@ export function prettyPrint(ast: AST, indent = 0): string {
 
     switch (ast.kind) {
         case 'literal':
-            return `${i}${ast.value.value}`;
+            return `${i}${ast.value}`;
 
         case 'variable':
             return `${i}${ast.name}`;
@@ -154,17 +154,56 @@ export function prettyPrint(ast: AST, indent = 0): string {
             return `${i}{${fields}}`;
 
         case 'lambda':
-            const params = ast.params.join(', ')
+            const params = ast.params.join(', ');
             return `${i}(${params}) => ${prettyPrint(ast.body)}`
 
         case 'record-access':
-            return `${i}${prettyPrint(ast.record)}.${ast.field}`
+            return `${i}${prettyPrint(ast.record)}.${ast.field}`;
 
         case 'record-update':
             const updates = Object.entries(ast.updates).map(([k, v]) => `${k} = ${prettyPrint(v, 0)}`).join(', ');
             return `${i}${prettyPrint(ast.record)} { ${updates} }`
 
+        case 'match':
+            const expr = prettyPrint(ast.expr, 0);
+            const cases = ast.cases
+                .map(c => `  | ${prettyPrintPattern(c.pattern)} -> ${prettyPrint(c.result, 0)}`)
+                .join('\n');
+            return `${i}match ${expr}\n${cases}`;
+
+
         default:
             return `${i}${ast.kind}`
     }
 }
+
+function prettyPrintPattern(pattern: Pattern): string {
+    switch (pattern.kind) {
+        case 'wildcard':
+            return '_';
+
+        case 'var':
+            return pattern.name;
+
+        case 'literal':
+            return JSON.stringify(pattern.value);
+
+        case 'constructor':
+            if (pattern.args.length === 0) {
+                return pattern.name;
+            }
+            const args = pattern.args.map(prettyPrintPattern).join(' ');
+            return `(${pattern.name} ${args})`;
+
+        case 'list':
+            const elems = pattern.elements.map(prettyPrintPattern).join(', ');
+            return `[${elems}]`;
+
+        case 'record':
+            const fields = Object.entries(pattern.fields)
+                .map(([k, p]) => `${k} = ${prettyPrintPattern(p)}`)
+                .join(', ');
+
+            return `{${fields}}`;
+    }
+}

src/lexer.ts

@@ -48,6 +48,7 @@ export function tokenize(source: string): Token[] {
             continue;
         }
 
+        // Comments
         if (char === '#') {
             while (i < source.length && source[i] !== '\n') {
                 i++;
@@ -76,7 +77,7 @@ export function tokenize(source: string): Token[] {
             continue;
         }
 
-        // Idents
+        // Idents & Wildcard
         if (/[A-Za-z_]/.test(char)) {
             let str = '';
             while (i < source.length && /[A-Za-z0-9_!-]/.test(source[i])) {
@@ -84,11 +85,16 @@ export function tokenize(source: string): Token[] {
                 i++;
             }
 
-            const isType = /[A-Z]/.test(str[0]);
+            if (str === '_') {
+                // Wildcards
+                tokens.push({ kind: 'underscore' });
+            } else {
+                const isType = /[A-Z]/.test(str[0]);
 
-            tokens.push(isType
+                tokens.push(isType
-                ? { kind: 'type-ident', value: str }
+                    ? { kind: 'type-ident', value: str }
-                : { kind: 'ident', value: str });
+                    : { kind: 'ident', value: str });
+            }
 
             continue;
         }

src/main.ts

@@ -5,11 +5,15 @@ import { Parser } from './parser'
 import { prettyPrint } from './ast';
 
 function e(str: string) {
+    console.log(str);
     const tokens = tokenize(str);
+    console.log(tokens);
     const p = new Parser(tokens);
     const ast = p.parse();
+    console.log(ast);
+    console.log(prettyPrint(ast));
     const env: Env = new Map();
-    console.log(str, tokens, prettyPrint(ast), evaluate(ast, env));
+    // console.log(evaluate(ast, env));
 }
 
 e('add1 = (x \\ x + 1); add1 3');
@@ -21,3 +25,6 @@ e('rec = { a = 3, b = 5 }; rec{ a = 10 }');
 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');

src/parser.ts

@@ -1,5 +1,5 @@
 import type { Token } from './lexer'
-import type { AST } from './ast'
+import type { AST, MatchCase, Pattern } from './ast'
 
 export class Parser {
     private tokens: Token[]
@@ -92,15 +92,128 @@ export class Parser {
     }
 
     private parseExpression(): AST {
+        // Lambda
         if (this.isLambdaStart()) {
             return this.parseLambda();
         }
 
+        // Let
         if (this.current().kind === 'ident' && this.peek().kind === 'equals') {
             return this.parseLet();
         }
 
-        return this.parseInfix();
+        let expr = this.parseInfix();
+
+        // Match
+        if (this.current().kind === 'pipe') {
+            return this.parseMatch(expr);
+        }
+
+        return expr;
+    }
+
+    private parseMatch(expr: AST): AST {
+        const cases: MatchCase[] = [];
+
+        while(this.current().kind === 'pipe') {
+            this.advance();
+            const pattern = this.parsePattern();
+            this.expect('backslash');
+            const result = this.parseInfix();
+            cases.push({ pattern, result })
+        }
+
+        return { kind: 'match', expr, cases };
+    }
+
+    private parsePattern(): Pattern {
+        const token = this.current();
+
+        // Wildcard
+        if (token.kind === 'underscore') {
+            this.advance();
+            return { kind: 'wildcard' };
+        }
+
+        // Literal
+        if (token.kind === 'int' || token.kind === 'float' || token.kind === 'string') {
+            this.advance();
+            return { kind: 'literal', value: token.value };
+        }
+
+        // Variable
+        if (token.kind === 'ident') {
+            this.advance();
+            return { kind: 'var', name: token.value };
+        }
+
+        // Constructor
+        if (token.kind === 'type-ident') {
+            this.advance();
+            const name = token.value;
+            const args: Pattern[] = [];
+
+            while (this.canStartPattern()) {
+                args.push(this.parsePattern());
+            }
+
+            return { kind: 'constructor', name, args };
+        }
+
+        // List
+        if (token.kind === 'open-bracket') {
+            this.advance();
+            const elements: Pattern[] = [];
+            let first = true;
+
+            while (this.current().kind !== 'close-bracket') {
+                if (!first) this.expect('comma');
+                first = false;
+                elements.push(this.parsePattern());
+            }
+
+            this.expect('close-bracket');
+            return { kind: 'list', elements };
+        }
+
+        // Record
+        if (token.kind === 'open-brace') {
+            this.advance();
+            const fields: { [key: string]: Pattern }= [];
+            let first = true;
+
+            while (this.current().kind !== 'close-brace') {
+                if (!first) this.expect('comma');
+                first = false;
+
+                const keyToken = this.expect('ident');
+                const key = (keyToken as { value: string }).value;
+                this.expect('equals');
+                fields[key] = this.parsePattern();
+            }
+
+            this.expect('close-brace');
+            return { kind: 'list', fields };
+        }
+
+        // Parens
+        if (token.kind === 'open-paren') {
+            this.advance();
+            const pattern = this.parsePattern();
+            this.expect('close-paren');
+            return pattern;
+        }
+
+        throw new Error(`Unexpected token in pattern: ${token.kind}`)
+    }
+
+    private canStartPattern(): boolean {
+        const kind = this.current().kind;
+
+        return kind === 'underscore' || kind === 'ident' ||
+            kind === 'type-ident' || kind === 'int' ||
+            kind === 'float' || kind === 'string' ||
+            kind === 'open-paren';
     }
 
     private parseLambda(): AST {