Delta: An Incremental Compiler
Step 9: If/Else statements

Description

Add support for if/else statements. This kind of statement should start with the if keyword, followed by an expression (the condition), followed by a list of zero or more statements enclosed inside curly braces (the then-block). An optional else clause can follow: an else keyword followed by a list of zero or more statements enclosed inside curly braces (the else-block). If the condition evaluates to true (any non-zero value) the then-block is executed, otherwise the else-block is executed if provided.

Make sure to consider if and else as Delta reserved keywords from now on.

Example

The Delta program:

var x, y;
x = false;
y = 5;
if !x {
    y = y * 2;
} else {
    y = y + 1;
}
y

should produce the following WAT code:

(module
  (func
    (export "_start")
    (result i32)
    (local $x i32)
    (local $y i32)
    i32.const 0
    local.set $x
    i32.const 5
    local.set $y
    local.get $x
    i32.eqz
    if
    local.get $y
    i32.const 2
    i32.mul
    local.set $y
    else
    local.get $y
    i32.const 1
    i32.add
    local.set $y
    end
    local.get $y
  )
)

The above WAT function’s return value should be:

10

Unit Tests

# File: tests/test_09_if_else.py

from unittest import TestCase
from delta import Compiler, SyntaxMistake
from delta.semantics import SemanticMistake


class TestIfElse(TestCase):

    def setUp(self):
        self.c = Compiler('program')

    def test_syntax_mistake(self):
        with self.assertRaises(SyntaxMistake):
            self.c.realize('if true {')

    def test_semantic_mistake1(self):
        with self.assertRaises(SemanticMistake):
            self.c.realize('var if; 0')

    def test_semantic_mistake2(self):
        with self.assertRaises(SemanticMistake):
            self.c.realize('var else; 0')

    def test_if(self):
        self.assertEqual(11,
                         self.c.realize(
                            '''
                            var x;
                            x = 10;
                            if true {
                                x = x + 1;
                            }
                            x
                            '''))

    def test_if_else_1(self):
        self.assertEqual(2,
                         self.c.realize(
                            '''
                            var x;
                            x = 1;
                            if true {
                                x = x * 2;
                            } else {
                                x = x + 2;
                            }
                            x
                            '''))

    def test_if_else_2(self):
        self.assertEqual(3,
                         self.c.realize(
                            '''
                            var x;
                            x = 1;
                            if false {
                                x = x * 2;
                            } else {
                                x = x + 2;
                            }
                            x
                            '''))

    def test_if_else_3(self):
        self.assertEqual(1,
                         self.c.realize(
                            '''
                            var x;
                            x = true;
                            if x {
                            } else {
                            }
                            x
                            '''))

    def test_if_else_4(self):
        self.assertEqual(0,
                         self.c.realize(
                            '''
                            var x;
                            x = false;
                            if x {
                            } else {
                            }
                            x
                            '''))

    def test_if_else_5(self):
        self.assertEqual(38,
                         self.c.realize(
                            '''
                            var a, b, c;
                            a = 1;
                            b = 2;
                            c = 3;
                            if a + b - c {
                                a = a * 2;
                                b = b * 3;
                                c = c * 4;
                            } else {
                                a = a * 5;
                                b = b * 6;
                                c = c * 7;
                            }
                            a + b + c
                            '''))

    def test_if_else_6(self):
        self.assertEqual(20,
                         self.c.realize(
                            '''
                            var a, b, c;
                            a = 1;
                            b = 2;
                            c = 3;
                            if -1 + a + b - c {
                                a = a * 2;
                                b = b * 3;
                                c = c * 4;
                            } else {
                                a = a * 5;
                                b = b * 6;
                                c = c * 7;
                            }
                            a + b + c
                            '''))