Implement if expressions

kyleect · kyleect · commit 032d4e82dc11 · 2026-05-18T23:27:44.000-05:00
diff --git a/docs/EXTENDING_LOX.md b/docs/EXTENDING_LOX.md
@@ -17,6 +17,7 @@ These are the documented changes to the language/syntax from the original Lox la
   - `strIsEmpty`
 - Block expressions: `var sum = { 5 + 10 };`
   - New bytecode op: `OP_CLOSE_BLOCK_EXPR n`
+- If expressions: `print if (true) "Hello" else "World"`
 - Native Functions
   - [ ] `arrSort(array, fn)`
   - [ ] `arrMap(array, fn)`
diff --git a/src/compiler.c b/src/compiler.c
@@ -642,6 +642,31 @@ static void ifStatement(Scanner *scanner) {
   patchJump(elseJump);
 }
 
+static void ifExpression(Scanner *scanner, bool canAssign) {
+  TRACELN("  compiler.ifExpression()");
+
+  consume(scanner, TOKEN_LEFT_PAREN, "Expect '(' after 'if'.");
+  expression(scanner);
+  consume(scanner, TOKEN_RIGHT_PAREN, "Expect ')' after condition.");
+
+  int thenJump = emitJump(OP_JUMP_IF_FALSE);
+  emitByte(OP_POP);
+  expression(scanner);
+
+  int elseJump = emitJump(OP_JUMP);
+
+  patchJump(thenJump);
+  emitByte(OP_POP);
+
+  if (match(scanner, TOKEN_ELSE)) {
+    expression(scanner);
+  } else {
+    emitByte(OP_NIL);
+  }
+
+  patchJump(elseJump);
+}
+
 static void printStatement(Scanner *scanner) {
   TRACELN("  compiler.printStatement()");
 
@@ -1121,7 +1146,7 @@ ParseRule rules[] = {
     [TOKEN_FALSE] = {literal, NULL, PREC_NONE},
     [TOKEN_FOR] = {NULL, NULL, PREC_NONE},
     [TOKEN_FUN] = {NULL, NULL, PREC_NONE},
-    [TOKEN_IF] = {NULL, NULL, PREC_NONE},
+    [TOKEN_IF] = {ifExpression, NULL, PREC_NONE},
     [TOKEN_NIL] = {literal, NULL, PREC_NONE},
     [TOKEN_OR] = {NULL, or_, PREC_OR},
     [TOKEN_QUESTION_QUESTION] = {NULL, nullish, PREC_OR},
diff --git a/tests/flow_control/if_expression_else.lox b/tests/flow_control/if_expression_else.lox
@@ -0,0 +1,11 @@
+print if (true) {
+    "Hello"
+} else {
+    "World"
+};
+
+print if (false) {
+    "Hello"
+} else {
+    "World"
+};
diff --git a/tests/flow_control/if_expression_else.lox.err b/tests/flow_control/if_expression_else.lox.err
@@ -0,0 +1,26 @@
+== <script> ==
+0000    1 OP_NIL
+0001    | OP_RETURN
+== <script> ==
+0000    1 OP_TRUE
+0001    | OP_JUMP_IF_FALSE    1 -> 12
+0004    | OP_POP
+0005    2 OP_CONSTANT         0 'Hello'
+0007    3 OP_CLOSE_BLOCK      0
+0009    | OP_JUMP             9 -> 17
+0012    | OP_POP
+0013    4 OP_CONSTANT         1 'World'
+0015    5 OP_CLOSE_BLOCK      0
+0017    | OP_PRINT
+0018    7 OP_FALSE
+0019    | OP_JUMP_IF_FALSE   19 -> 30
+0022    | OP_POP
+0023    8 OP_CONSTANT         2 'Hello'
+0025    9 OP_CLOSE_BLOCK      0
+0027    | OP_JUMP            27 -> 35
+0030    | OP_POP
+0031   10 OP_CONSTANT         3 'World'
+0033   11 OP_CLOSE_BLOCK      0
+0035    | OP_PRINT
+0036    | OP_NIL
+0037    | OP_RETURN
diff --git a/tests/flow_control/if_expression_else.lox.exit b/tests/flow_control/if_expression_else.lox.exit
@@ -0,0 +1 @@
+0
diff --git a/tests/flow_control/if_expression_else.lox.out b/tests/flow_control/if_expression_else.lox.out
@@ -0,0 +1,2 @@
+Hello
+World
diff --git a/tests/flow_control/if_expression_else_branch_expressions.lox b/tests/flow_control/if_expression_else_branch_expressions.lox
@@ -0,0 +1,3 @@
+print if (true) "Hello" else "World";
+
+print if (false) "Hello" else "World";
diff --git a/tests/flow_control/if_expression_else_branch_expressions.lox.err b/tests/flow_control/if_expression_else_branch_expressions.lox.err
@@ -0,0 +1,22 @@
+== <script> ==
+0000    1 OP_NIL
+0001    | OP_RETURN
+== <script> ==
+0000    1 OP_TRUE
+0001    | OP_JUMP_IF_FALSE    1 -> 10
+0004    | OP_POP
+0005    | OP_CONSTANT         0 'Hello'
+0007    | OP_JUMP             7 -> 13
+0010    | OP_POP
+0011    | OP_CONSTANT         1 'World'
+0013    | OP_PRINT
+0014    3 OP_FALSE
+0015    | OP_JUMP_IF_FALSE   15 -> 24
+0018    | OP_POP
+0019    | OP_CONSTANT         2 'Hello'
+0021    | OP_JUMP            21 -> 27
+0024    | OP_POP
+0025    | OP_CONSTANT         3 'World'
+0027    | OP_PRINT
+0028    | OP_NIL
+0029    | OP_RETURN
diff --git a/tests/flow_control/if_expression_else_branch_expressions.lox.exit b/tests/flow_control/if_expression_else_branch_expressions.lox.exit
@@ -0,0 +1 @@
+0
diff --git a/tests/flow_control/if_expression_else_branch_expressions.lox.out b/tests/flow_control/if_expression_else_branch_expressions.lox.out
@@ -0,0 +1,2 @@
+Hello
+World
diff --git a/tests/flow_control/if_expression_else_if.lox b/tests/flow_control/if_expression_else_if.lox
@@ -0,0 +1,13 @@
+fun ifelseifelse (a, b) {
+    print if (a) {
+        "Hello"
+    } else if (b) {
+        "World"
+    } else {
+        "Goodbye"
+    };
+}
+
+ifelseifelse (true, false);
+ifelseifelse (false, true);
+ifelseifelse (false, false);
diff --git a/tests/flow_control/if_expression_else_if.lox.err b/tests/flow_control/if_expression_else_if.lox.err
@@ -0,0 +1,43 @@
+== <script> ==
+0000    1 OP_NIL
+0001    | OP_RETURN
+== ifelseifelse ==
+0000    2 OP_GET_LOCAL        1
+0002    | OP_JUMP_IF_FALSE    2 -> 13
+0005    | OP_POP
+0006    3 OP_CONSTANT         0 'Hello'
+0008    4 OP_CLOSE_BLOCK      0
+0010    | OP_JUMP            10 -> 32
+0013    | OP_POP
+0014    | OP_GET_LOCAL        2
+0016    | OP_JUMP_IF_FALSE   16 -> 27
+0019    | OP_POP
+0020    5 OP_CONSTANT         1 'World'
+0022    6 OP_CLOSE_BLOCK      0
+0024    | OP_JUMP            24 -> 32
+0027    | OP_POP
+0028    7 OP_CONSTANT         2 'Goodbye'
+0030    8 OP_CLOSE_BLOCK      0
+0032    | OP_PRINT
+0033    9 OP_NIL
+0034    | OP_RETURN
+== <script> ==
+0000    9 OP_CLOSURE          1 <fn ifelseifelse>
+0002    | OP_DEFINE_GLOBAL    0 'ifelseifelse'
+0004   11 OP_GET_GLOBAL       2 'ifelseifelse'
+0006    | OP_TRUE
+0007    | OP_FALSE
+0008    | OP_CALL             2
+0010    | OP_POP
+0011   12 OP_GET_GLOBAL       3 'ifelseifelse'
+0013    | OP_FALSE
+0014    | OP_TRUE
+0015    | OP_CALL             2
+0017    | OP_POP
+0018   13 OP_GET_GLOBAL       4 'ifelseifelse'
+0020    | OP_FALSE
+0021    | OP_FALSE
+0022    | OP_CALL             2
+0024    | OP_POP
+0025   14 OP_NIL
+0026    | OP_RETURN
diff --git a/tests/flow_control/if_expression_else_if.lox.exit b/tests/flow_control/if_expression_else_if.lox.exit
@@ -0,0 +1 @@
+0
diff --git a/tests/flow_control/if_expression_else_if.lox.out b/tests/flow_control/if_expression_else_if.lox.out
@@ -0,0 +1,3 @@
+Hello
+World
+Goodbye
diff --git a/tests/flow_control/if_expression_else_if_branch_expressions.lox b/tests/flow_control/if_expression_else_if_branch_expressions.lox
@@ -0,0 +1,13 @@
+fun ifelseifelse (a, b) {
+    print
+        if (a)
+            "Hello"
+        else if (b)
+            "World"
+        else
+            "Goodbye";
+}
+
+ifelseifelse (true, false);
+ifelseifelse (false, true);
+ifelseifelse (false, false);
diff --git a/tests/flow_control/if_expression_else_if_branch_expressions.lox.err b/tests/flow_control/if_expression_else_if_branch_expressions.lox.err
@@ -0,0 +1,40 @@
+== <script> ==
+0000    1 OP_NIL
+0001    | OP_RETURN
+== ifelseifelse ==
+0000    3 OP_GET_LOCAL        1
+0002    | OP_JUMP_IF_FALSE    2 -> 11
+0005    | OP_POP
+0006    4 OP_CONSTANT         0 'Hello'
+0008    | OP_JUMP             8 -> 26
+0011    | OP_POP
+0012    5 OP_GET_LOCAL        2
+0014    | OP_JUMP_IF_FALSE   14 -> 23
+0017    | OP_POP
+0018    6 OP_CONSTANT         1 'World'
+0020    | OP_JUMP            20 -> 26
+0023    | OP_POP
+0024    8 OP_CONSTANT         2 'Goodbye'
+0026    | OP_PRINT
+0027    9 OP_NIL
+0028    | OP_RETURN
+== <script> ==
+0000    9 OP_CLOSURE          1 <fn ifelseifelse>
+0002    | OP_DEFINE_GLOBAL    0 'ifelseifelse'
+0004   11 OP_GET_GLOBAL       2 'ifelseifelse'
+0006    | OP_TRUE
+0007    | OP_FALSE
+0008    | OP_CALL             2
+0010    | OP_POP
+0011   12 OP_GET_GLOBAL       3 'ifelseifelse'
+0013    | OP_FALSE
+0014    | OP_TRUE
+0015    | OP_CALL             2
+0017    | OP_POP
+0018   13 OP_GET_GLOBAL       4 'ifelseifelse'
+0020    | OP_FALSE
+0021    | OP_FALSE
+0022    | OP_CALL             2
+0024    | OP_POP
+0025   14 OP_NIL
+0026    | OP_RETURN
diff --git a/tests/flow_control/if_expression_else_if_branch_expressions.lox.exit b/tests/flow_control/if_expression_else_if_branch_expressions.lox.exit
@@ -0,0 +1 @@
+0
diff --git a/tests/flow_control/if_expression_else_if_branch_expressions.lox.out b/tests/flow_control/if_expression_else_if_branch_expressions.lox.out
@@ -0,0 +1,3 @@
+Hello
+World
+Goodbye
diff --git a/tests/flow_control/if_expression_no_else.lox b/tests/flow_control/if_expression_no_else.lox
@@ -0,0 +1,7 @@
+print if (true) {
+    "Hello"
+};
+
+print if (false) {
+    "World"
+};
diff --git a/tests/flow_control/if_expression_no_else.lox.err b/tests/flow_control/if_expression_no_else.lox.err
@@ -0,0 +1,24 @@
+== <script> ==
+0000    1 OP_NIL
+0001    | OP_RETURN
+== <script> ==
+0000    1 OP_TRUE
+0001    | OP_JUMP_IF_FALSE    1 -> 12
+0004    | OP_POP
+0005    2 OP_CONSTANT         0 'Hello'
+0007    3 OP_CLOSE_BLOCK      0
+0009    | OP_JUMP             9 -> 14
+0012    | OP_POP
+0013    | OP_NIL
+0014    | OP_PRINT
+0015    5 OP_FALSE
+0016    | OP_JUMP_IF_FALSE   16 -> 27
+0019    | OP_POP
+0020    6 OP_CONSTANT         1 'World'
+0022    7 OP_CLOSE_BLOCK      0
+0024    | OP_JUMP            24 -> 29
+0027    | OP_POP
+0028    | OP_NIL
+0029    | OP_PRINT
+0030    8 OP_NIL
+0031    | OP_RETURN
diff --git a/tests/flow_control/if_expression_no_else.lox.exit b/tests/flow_control/if_expression_no_else.lox.exit
@@ -0,0 +1 @@
+0
diff --git a/tests/flow_control/if_expression_no_else.lox.out b/tests/flow_control/if_expression_no_else.lox.out
@@ -0,0 +1,2 @@
+Hello
+nil

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	`+print if (true) "Hello" else "World";`
	`2`	`+`
	`3`	`+print if (false) "Hello" else "World";`