Implement implicit returns in functions by excluding semicolon

Author: kyleect

docs/EXTENDING_LOX.md

@@ -6,6 +6,7 @@ These are the documented changes to the language/syntax from the original Lox la
 
 - Support to escape characters in strings: `\n`, `\r`, `\t`, `\"`, `\\`
 - Function body expressions: `fun sum(a, b) = a + b;`
+- Function implicit returns: `fun sum(a, b) { a + b }`
 - Native Functions
   - [ ] `arrSort(array, fn)`
   - [ ] `arrMap(array, fn)`

examples/fib.lox

@@ -1,6 +1,7 @@
 fun fib(n) {
   if (n < 2) return n;
-  return fib(n - 2) + fib(n - 1);
+
+  fib(n - 2) + fib(n - 1)
 }
 
 print "Fibonacci";

examples/zoo.lox

@@ -14,12 +14,12 @@ class Zoo {
     this.elephant = 1;
     this.fox      = 1;
   }
-  ant()    { return this.aardvark; }
-  banana() { return this.baboon; }
-  tuna()   { return this.cat; }
-  hay()    { return this.donkey; }
-  grass()  { return this.elephant; }
-  mouse()  { return this.fox; }
+  ant()    { this.aardvark }
+  banana() { this.baboon }
+  tuna()   { this.cat }
+  hay()    { this.donkey }
+  grass()  { this.elephant }
+  mouse()  { this.fox }
 }
 
 var zoo = Zoo();

src/compiler.c

@@ -49,6 +49,7 @@ static void endScope();
 static void classDeclaration(Scanner *scanner);
 static void namedVariable(Scanner *scanner, Token name, bool canAssign);
 static void indexValue(Scanner *scanner, bool canAssign);
+static void error(const char *message);
 
 Parser parser;
 Chunk *compilingChunk;
@@ -152,7 +153,19 @@ static void statement(Scanner *scanner) {
     block(scanner);
     endScope();
   } else {
-    expressionStatement(scanner);
+    expression(scanner);
+
+    if (match(scanner, TOKEN_SEMICOLON)) {
+      emitByte(OP_POP);
+      return;
+    }
+
+    if (current->type != TYPE_SCRIPT && check(TOKEN_RIGHT_BRACE)) {
+      emitByte(OP_RETURN);
+      return;
+    }
+
+    error("Expect ';' after expression.");
   }
 }
 

tests/assignments/assignment_edge_case.lox.err

@@ -65,4 +65,4 @@
 0028   14 OP_NIL
 0029    | OP_RETURN
 [line 6] Error at '=' (TOKEN_EQUAL): Invalid assignment target.
-[line 6] Error at 'c' (TOKEN_IDENTIFIER): Expect ';' after expression.
+[line 6] Error at '=' (TOKEN_EQUAL): Expect ';' after expression.

tests/errors/compiler_error_missing_semicolon.lox.err

@@ -64,4 +64,4 @@
 0026    | OP_DEFINE_GLOBAL   12 'is_empty'
 0028   14 OP_NIL
 0029    | OP_RETURN
-[line 1] Error at end: Expect ';' after expression.
+[line 1] Error at '1' (TOKEN_NUMBER): Expect ';' after expression.

tests/functions/function_implicit_return.lox

@@ -0,0 +1,3 @@
+fun sum(a, b) { a + b }
+
+print sum(1, 2);

tests/functions/function_implicit_return.lox.err

@@ -0,0 +1,81 @@
+== is ==
+0000    1 OP_GET_GLOBAL       0 'typeof'
+0002    | OP_GET_LOCAL        1
+0004    | OP_CALL             1
+0006    | OP_GET_LOCAL        2
+0008    | OP_EQUAL
+0009    | OP_RETURN
+== is_function ==
+0000    3 OP_GET_GLOBAL       0 'is'
+0002    | OP_GET_LOCAL        1
+0004    | OP_CONSTANT         1 'function'
+0006    | OP_CALL             2
+0008    | OP_RETURN
+== is_number ==
+0000    5 OP_GET_GLOBAL       0 'is'
+0002    | OP_GET_LOCAL        1
+0004    | OP_CONSTANT         1 'number'
+0006    | OP_CALL             2
+0008    | OP_RETURN
+== is_string ==
+0000    7 OP_GET_GLOBAL       0 'is'
+0002    | OP_GET_LOCAL        1
+0004    | OP_CONSTANT         1 'string'
+0006    | OP_CALL             2
+0008    | OP_RETURN
+== is_bool ==
+0000    9 OP_GET_GLOBAL       0 'is'
+0002    | OP_GET_LOCAL        1
+0004    | OP_CONSTANT         1 'bool'
+0006    | OP_CALL             2
+0008    | OP_RETURN
+== is_nil ==
+0000   11 OP_GET_GLOBAL       0 'is'
+0002    | OP_GET_LOCAL        1
+0004    | OP_CONSTANT         1 'nil'
+0006    | OP_CALL             2
+0008    | OP_RETURN
+== is_empty ==
+0000   13 OP_GET_GLOBAL       0 'is_string'
+0002    | OP_GET_LOCAL        1
+0004    | OP_CALL             1
+0006    | OP_JUMP_IF_FALSE    6 -> 19
+0009    | OP_POP
+0010    | OP_GET_GLOBAL       1 'len'
+0012    | OP_GET_LOCAL        1
+0014    | OP_CALL             1
+0016    | OP_CONSTANT         2 '0.000000'
+0018    | OP_EQUAL
+0019    | OP_RETURN
+== <script> ==
+0000    1 OP_CLOSURE          1 <fn is>
+0002    | OP_DEFINE_GLOBAL    0 'is'
+0004    3 OP_CLOSURE          3 <fn is_function>
+0006    | OP_DEFINE_GLOBAL    2 'is_function'
+0008    5 OP_CLOSURE          5 <fn is_number>
+0010    | OP_DEFINE_GLOBAL    4 'is_number'
+0012    7 OP_CLOSURE          7 <fn is_string>
+0014    | OP_DEFINE_GLOBAL    6 'is_string'
+0016    9 OP_CLOSURE          9 <fn is_bool>
+0018    | OP_DEFINE_GLOBAL    8 'is_bool'
+0020   11 OP_CLOSURE         11 <fn is_nil>
+0022    | OP_DEFINE_GLOBAL   10 'is_nil'
+0024   13 OP_CLOSURE         13 <fn is_empty>
+0026    | OP_DEFINE_GLOBAL   12 'is_empty'
+0028   14 OP_NIL
+0029    | OP_RETURN
+== sum ==
+0000    1 OP_GET_LOCAL        1
+0002    | OP_GET_LOCAL        2
+0004    | OP_ADD
+0005    | OP_RETURN
+== <script> ==
+0000    1 OP_CLOSURE          1 <fn sum>
+0002    | OP_DEFINE_GLOBAL    0 'sum'
+0004    3 OP_GET_GLOBAL       2 'sum'
+0006    | OP_CONSTANT         3 '1.000000'
+0008    | OP_CONSTANT         4 '2.000000'
+0010    | OP_CALL             2
+0012    | OP_PRINT
+0013    4 OP_NIL
+0014    | OP_RETURN

tests/functions/function_implicit_return.lox.exit

@@ -0,0 +1 @@
+0

tests/functions/function_implicit_return.lox.out

@@ -0,0 +1 @@
+3.000000