Delta: An Incremental Compiler
Step 8: Variables

Description

Add support for variables, specifically:

• Variable declaration statement: The var keyword followed by a coma separated list of variable names ending with a semicolon. Examples:

  var some_var;
  

  var x, y, z;
  

  A variable name should start with an upper or lower case letter followed by zero or more upper or lower case letters, digits, or underscore characters.

  Variable names and keywords should be case-sensitive.

  A delta.SemanticMistake exception should be raised when trying to declare a variable that has already been declared or when trying to name a variable using a Delta language reserved keyword (these are so far: true, false, and var).

  It’s worth noting that WebAssembly local variables are initialized with zero by default once they are declared.

• Variable assignment statement: A variable name followed by an equal sign (=) followed by an expression and ending with a semicolon. Example:

  some_var = 1 + 2 * 3;
  

  A delta.SemanticMistake exception should be raised when trying to assign to a variable that hasn’t been declared.

• Variable references: A variable name should be allowed to be used as part of any expression. Example:

  2 * x + 1
  

  A delta.SemanticMistake exception should be raised when trying to reference a variable that hasn’t been declared.

A Delta program should be comprised of zero or more statements ending with an expression. See example below.

Example

The Delta program:

var x, y;         // First statement
x = 2;
y = (x + 1) * 3;
var z;
z = y - 1;        // Last statement
x + y + z         // Resulting expression

should produce the following WAT code:

(module
  (func
    (export "_start")
    (result i32)
    (local $x i32)
    (local $y i32)
    (local $z i32)
    i32.const 2
    local.set $x
    local.get $x
    i32.const 1
    i32.add
    i32.const 3
    i32.mul
    local.set $y
    local.get $y
    i32.const 1
    i32.sub
    local.set $z
    local.get $x
    local.get $y
    i32.add
    local.get $z
    i32.add
  )
)

The above WAT function’s return value should be:

19

Unit Tests

# File: tests/test_08_variables.py

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


class TestVariables(TestCase):

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

    def test_syntax_mistake(self):
        with self.assertRaises(SyntaxMistake):
            self.c.realize('var x,')

    def test_vars0(self):
        self.assertEqual(0,
                         self.c.realize(
                            '''
                            var var_1;
                            var var_2;
                            var var_3;
                            var_1 + var_2 + var_3
                            '''))

    def test_vars1(self):
        self.assertEqual(42,
                         self.c.realize(
                            '''
                            var x;
                            x = 42;
                            x
                            '''))

    def test_vars2(self):
        self.assertEqual(15,
                         self.c.realize(
                            '''
                            var x, y, z;
                            x = 2;
                            y = 8;
                            z = x * y - 1;
                            z
                            '''))

    def test_vars3(self):
        self.assertEqual(10,
                         self.c.realize(
                            '''
                            var a;
                            a = 10;
                            var b, c;
                            b = 100;
                            c = (b - a * 2) / 8;
                            c
                            '''))

    def test_vars4(self):
        self.assertEqual(55,
                         self.c.realize(
                            '''
                            var v, w, x, y, z;
                            v = 1;
                            w = 2;
                            x = 3;
                            y = 4;
                            z = 5;
                            var a, b, c, d, e;
                            a = 6;
                            b = 7;
                            c = 8;
                            d = 9;
                            e = 10;
                            a + b + c + d + e + v + w + x + y + z
                            '''))

    def test_vars5(self):
        self.assertEqual(19,
                         self.c.realize(
                            '''
                            var x, y;
                            x = 2;
                            y = (x + 1) * 3;
                            var z;
                            z = y - 1;
                            x + y + z
                            '''))

    def test_duplicated_variable1(self):
        with self.assertRaises(SemanticMistake):
            self.c.realize(
                '''
                var x, y, z;
                x = 10;
                var v, w, x;
                x
                ''')

    def test_duplicated_variable2(self):
        with self.assertRaises(SemanticMistake):
            self.c.realize(
                '''
                var x, y, x;
                x = 10;
                x
                ''')

    def test_reserved_word(self):
        with self.assertRaises(SemanticMistake):
            self.c.realize(
                '''
                var x, y, var;
                x = 1;
                y = 2;
                var = 3;
                x + y + var
                ''')

    def test_undeclared_lhs(self):
        with self.assertRaises(SemanticMistake):
            self.c.realize(
                '''
                var x;
                y = 10;
                x
                ''')

    def test_undeclared_rhs(self):
        with self.assertRaises(SemanticMistake):
            self.c.realize(
                '''
                var x;
                x = 10 + z;
                x
                ''')