Add support for AssignStatement in lexical analysis and CST

src/cst.ts

@@ -168,6 +168,7 @@ export const enum SyntaxKind {
   ReturnStatement,
   ExpressionStatement,
   IfStatement,
+  AssignStatement,
 
   // If statement elements
   IfStatementCase,
@@ -2235,6 +2236,36 @@ export class ReturnStatement extends SyntaxBase {
 
 }
 
+export class AssignStatement extends SyntaxBase {
+
+  public readonly kind = SyntaxKind.AssignStatement;
+
+  public constructor(
+    public pattern: Pattern,
+    public operator: Assignment,
+    public expression: Expression,
+  ) {
+    super();
+  }
+
+  public clone(): AssignStatement {
+    return new AssignStatement(
+      this.pattern.clone(),
+      this.operator.clone(),
+      this.expression.clone()
+    );
+  }
+
+  public getFirstToken(): Token {
+    return this.pattern.getFirstToken();
+  }
+
+  public getLastToken(): Token {
+    return this.expression.getLastToken();
+  }
+
+}
+
 export class ExpressionStatement extends SyntaxBase {
 
   public readonly kind = SyntaxKind.ExpressionStatement;
@@ -2263,6 +2294,7 @@ export type Statement
   = ReturnStatement
   | ExpressionStatement
   | IfStatement
+  | AssignStatement
 
 export class Param extends SyntaxBase {
 

src/parser.ts

@@ -66,6 +66,7 @@ import {
   ClassKeyword,
   InstanceDeclaration,
   ClassConstraintClause,
+  AssignStatement,
 } from "./cst"
 import { Stream } from "./util";
 
@@ -782,6 +783,27 @@ export class Parser {
     return new Param(pattern);
   }
 
+  private lookaheadIsAssignment(): boolean {
+    for (let i = 1;; i++) {
+      const t0 = this.peekToken(i);
+      switch (t0.kind) {
+        case SyntaxKind.LineFoldEnd:
+        case SyntaxKind.BlockStart:
+          return false;
+        case SyntaxKind.Assignment:
+          return true;
+      }
+    }
+  }
+
+  public parseAssignStatement(): AssignStatement {
+    const pattern = this.parsePattern();
+    const operator = this.expectToken(SyntaxKind.Assignment);
+    const expression = this.parseExpression();
+    this.expectToken(SyntaxKind.LineFoldEnd);
+    return new AssignStatement(pattern, operator, expression);
+  }
+
   public parseLetBodyElement(): LetBodyElement {
     const t0 = this.peekTokenAfterModifiers();
     switch (t0.kind) {
@@ -792,6 +814,9 @@ export class Parser {
       case SyntaxKind.IfKeyword:
         return this.parseIfStatement();
       default:
+        if (this.lookaheadIsAssignment()) {
+          return this.parseAssignStatement();
+        }
         // TODO convert parse errors to include LetKeyword and ReturnKeyword
         return this.parseExpressionStatement();
     }
@@ -1165,6 +1190,9 @@ export class Parser {
       case SyntaxKind.IfKeyword:
         return this.parseIfStatement();
       default:
+        if (this.lookaheadIsAssignment()) {
+          return this.parseAssignStatement();
+        }
         return this.parseExpressionStatement();
     }
   }

src/scanner.ts

@@ -229,7 +229,15 @@ export class Scanner extends BufferedStream<Token> {
         case '{': return new LBrace(startPos);
         case '}': return new RBrace(startPos);
         case ',': return new Comma(startPos);
-        case ':': return new Colon(startPos);
+        case ':':
+          const text = this.takeWhile(isOperatorPart);
+          if (text === '') {
+            return new Colon(startPos);
+          } else if (text === '=') {
+            return new Assignment(':', startPos);
+          } else {
+            throw new ScanError(this.file, startPos, ':' + text);
+          }
         case '.': {
           const dots = c0 + this.takeWhile(ch => ch === '.');
           if (dots === '.') {
@@ -266,7 +274,7 @@ export class Scanner extends BufferedStream<Token> {
           } else if (text === '=') {
             return new Equals(startPos);
           } else if (text.endsWith('=') && text[text.length-2] !== '=') {
-            return new Assignment(text, startPos);
+            return new Assignment(text.substring(0, text.length-1), startPos);
           } else {
             return new CustomOperator(text, startPos);
           }