def test_calling_atom_raises_exception():
"""A function call to a non-function should result in an error."""
with assert_raises_regexp(LispError, "not a function"):
evaluate(parse("(#t 'foo 'bar)"), Environment())
with assert_raises_regexp(LispError, "not a function"):
evaluate(parse("(42)"), Environment())
def test_lookup_from_inner_env():
"""The `extend` function returns a new environment extended with more bindings."""
env = Environment({"foo": 42})
env = env.extend({"bar": True})
assert_equals(42, env.lookup("foo"))
assert_equals(True, env.lookup("bar"))
def test_lambda_arguments_are_lists():
"""The parameters of a `lambda` should be a list."""
closure = evaluate(parse("(lambda (x y) (+ x y))"), Environment())
assert_true(is_list(closure.params))
with assert_raises(LispError):
evaluate(parse("(lambda not-a-list (body of fn))"), Environment())
def test_checking_whether_list_is_empty():
"""The `empty` form checks whether or not a list is empty."""
assert_equals(False, evaluate(parse("(empty '(1 2 3))"), Environment()))
assert_equals(False, evaluate(parse("(empty '(1))"), Environment()))
assert_equals(True, evaluate(parse("(empty '())"), Environment()))
assert_equals(True, evaluate(parse("(empty (tail '(1)))"), Environment()))
def test_extend_returns_new_environment():
"""The extend method should create a new environment, leaving the old one unchanged."""
env = Environment({"foo": 1})
extended = env.extend({"foo": 2})
assert_equals(1, env.lookup("foo"))
assert_equals(2, extended.lookup("foo"))
def test_define_with_wrong_number_of_arguments():
"""Defines should have exactly two arguments, or raise an error"""
with assert_raises_regexp(LispError, "Wrong number of arguments"):
evaluate(parse("(define x)"), Environment())
with assert_raises_regexp(LispError, "Wrong number of arguments"):
evaluate(parse("(define x 1 2)"), Environment())
def test_lookup_on_missing_raises_exception():
"""When looking up an undefined symbol, an error should be raised.
The error message should contain the relevant symbol, and inform that it has
not been defined."""
with assert_raises_regexp(LispError, "my-missing-var"):
empty_env = Environment()
empty_env.lookup("my-missing-var")
def test_evaluating_quote():
"""When a call is done to the `quote` form, the argument should be returned without
being evaluated.
(quote foo) -> foo
"""
assert_equals("foo", evaluate(["quote", "foo"], Environment()))
assert_equals([1, 2, False],
evaluate(["quote", [1, 2, False]], Environment()))
def test_redefine_variables_illegal():
"""Variables can only be defined once.
Setting a variable in an environment where it is already defined should result
in an appropriate error.
"""
env = Environment({"foo": 1})
with assert_raises_regexp(LispError, "already defined"):
env.set("foo", 2)
def test_evaluating_call_to_closure():
"""The first case we'll handle is when the AST is a list with an actual closure
as the first element.
In this first test, we'll start with a closure with no arguments and no free
variables. All we need to do is to evaluate and return the function body."""
closure = evaluate(parse("(lambda () (+ 1 2))"), Environment())
ast = [closure]
result = evaluate(ast, Environment())
assert_equals(3, result)
def test_lookup_deeply_nested_var():
"""Extending overwrites old bindings to the same variable name."""
env = Environment({
"a": 1
}).extend({
"b": 2
}).extend({
"c": 3
}).extend({"foo": 100})
assert_equals(100, env.lookup("foo"))
def test_define():
"""Test of simple define statement.
The `define` form is used to define new bindings in the environment.
A `define` call should result in a change in the environment. What you
return from evaluating the definition is not important (although it
affects what is printed in the REPL)."""
env = Environment()
evaluate(parse("(define x 1000)"), env)
assert_equals(1000, env.lookup("x"))
def test_math_operators_only_work_on_numbers():
"""The math functions should only allow numbers as arguments."""
with assert_raises(LispError):
evaluate(parse("(+ 1 'foo)"), Environment())
with assert_raises(LispError):
evaluate(parse("(- 1 'foo)"), Environment())
with assert_raises(LispError):
evaluate(parse("(/ 1 'foo)"), Environment())
with assert_raises(LispError):
evaluate(parse("(mod 1 'foo)"), Environment())
def test_evaluating_atom_function():
"""The `atom` form is used to determine whether an expression is an atom.
Atoms are expressions that are not list, i.e. integers, booleans or symbols.
Remember that the argument to `atom` must be evaluated before the check is done.
"""
assert_equals(True, evaluate(["atom", True], Environment()))
assert_equals(True, evaluate(["atom", False], Environment()))
assert_equals(True, evaluate(["atom", 42], Environment()))
assert_equals(True, evaluate(["atom", ["quote", "foo"]], Environment()))
assert_equals(False, evaluate(["atom", ["quote", [1, 2]]], Environment()))
def test_evaluating_call_to_closure_with_free_variables():
"""The body should be evaluated in the environment from the closure.
The function's free variables, i.e. those not specified as part of the parameter list,
should be looked up in the environment from where the function was defined. This is
the environment included in the closure. Make sure this environment is used when
evaluating the body."""
closure = evaluate(parse("(lambda (x) (+ x y))"), Environment({"y": 1}))
ast = [closure, 0]
result = evaluate(ast, Environment({"y": 2}))
assert_equals(1, result)
def test_calling_very_simple_function_in_environment():
"""A call to a symbol corresponds to a call to its value in the environment.
When a symbol is the first element of the AST list, it is resolved to its value in
the environment (which should be a function closure). An AST with the variables
replaced with its value should then be evaluated instead."""
env = Environment()
evaluate(parse("(define add (lambda (x y) (+ x y)))"), env)
assert_is_instance(env.lookup("add"), Closure)
result = evaluate(parse("(add 1 2)"), env)
assert_equals(3, result)
def test_calling_very_simple_function_in_environment():
"""A call to a symbol corresponds to a call to its value in the environment.
When a symbol is the first element of the AST list, it is resolved to its value in
the environment (which should be a function closure). An AST with the variables
replaced with its value should then be evaluated instead."""
env = Environment()
evaluate(parse("(define add (lambda (x y) (+ x y)))"), env)
assert_is_instance(env.lookup("add"), Closure)
result = evaluate(parse("(add 1 2)"), env)
assert_equals(3, result)
def test_basic_if_statement():
"""If statements are the basic control structures.
The `if` should first evaluate its first argument. If this evaluates to true, then
the second argument is evaluated and returned. Otherwise the third and last argument
is evaluated and returned instead."""
if_expression = parse("(if #t 42 1000)")
assert_equals(42, evaluate(if_expression, Environment()))
if_expression = parse("(if #f 42 1000)")
assert_equals(1000, evaluate(if_expression, Environment()))
if_expression = parse("(if #t #t #f)")
assert_equals(True, evaluate(if_expression, Environment()))
def test_lambda_closure_holds_function():
"""The closure contains the parameter list and function body too."""
closure = evaluate(parse("(lambda (x y) (+ x y))"), Environment())
assert_equals(["x", "y"], closure.params)
assert_equals(["+", "x", "y"], closure.body)
def test_lookup_missing_variable():
"""Referencing undefined variables should raise an appropriate exception.
This test should already be working if you implemented the environment correctly."""
with assert_raises_regexp(LispError, "my-var"):
evaluate("my-var", Environment())
def test_evaluating_eq_function():
"""The `eq` form is used to check whether two expressions are the same atom."""
assert_equals(True, evaluate(["eq", 1, 1], Environment()))
assert_equals(False, evaluate(["eq", 1, 2], Environment()))
# From this point, the ASTs might sometimes be too long or cummbersome to
# write down explicitly, and we'll use `parse` to make them for us.
# Remember, if you need to have a look at exactly what is passed to `evaluate`,
# just add a print statement in the test (or in `evaluate`).
assert_equals(True, evaluate(parse("(eq 'foo 'foo)"), Environment()))
assert_equals(False, evaluate(parse("(eq 'foo 'bar)"), Environment()))
# Lists are never equal, because lists are not atoms
assert_equals(False,
evaluate(parse("(eq '(1 2 3) '(1 2 3))"), Environment()))
def test_variable_lookup_after_define():
"""Test define and lookup variable in same environment.
This test should already be working when the above ones are passing."""
env = Environment()
evaluate(parse("(define foo (+ 2 2))"), env)
assert_equals(4, evaluate("foo", env))
def test_evaluating_symbol():
"""Symbols (other than #t and #f) are treated as variable references.
When evaluating a symbol, the corresponding value should be looked up in the
environment."""
env = Environment({"foo": 42})
assert_equals(42, evaluate("foo", env))
def test_calling_with_wrong_number_of_arguments():
"""Functions should raise exceptions when called with wrong number of arguments."""
env = Environment()
evaluate(parse("(define fn (lambda (p1 p2) 'whatever))"), env)
error_msg = "wrong number of arguments, expected 2 got 3"
with assert_raises_regexp(LispError, error_msg):
evaluate(parse("(fn 1 2 3)"), env)
def test_creating_longer_lists_with_only_cons():
"""`cons` needs to evaluate it's arguments.
Like all the other special forms and functions in our language, `cons` is
call-by-value. This means that the arguments must be evaluated before we
create the list with their values."""
result = evaluate(parse("(cons 3 (cons (- 4 2) (cons 1 '())))"), Environment())
assert_equals(parse("(3 2 1)"), result)
def test_calling_lambda_directly():
"""It should be possible to define and call functions directly.
A lambda definition in the call position of an AST should be evaluated, and then
evaluated as before."""
ast = parse("((lambda (x) x) 42)")
result = evaluate(ast, Environment())
assert_equals(42, result)
def test_lambda_closure_keeps_defining_env():
"""The closure should keep a copy of the environment where it was defined.
Once we start calling functions later, we'll need access to the environment
from when the function was created in order to resolve all free variables."""
env = Environment({"foo": 1, "bar": 2})
ast = ["lambda", [], 42]
closure = evaluate(ast, env)
assert_equals(closure.env, env)
def test_if_with_sub_expressions():
"""A final test with a more complex if expression.
This test should already be passing if the above ones are."""
if_expression = parse("""
(if (> 1 2)
(- 1000 1)
(+ 40 (- 3 1)))
""")
assert_equals(42, evaluate(if_expression, Environment()))
def test_basic_math_operators():
"""To be able to do anything useful, we need some basic math operators.
Since we only operate with integers, `/` must represent integer division.
`mod` is the modulo operator.
"""
assert_equals(4, evaluate(["+", 2, 2], Environment()))
assert_equals(1, evaluate(["-", 2, 1], Environment()))
assert_equals(3, evaluate(["/", 6, 2], Environment()))
assert_equals(3, evaluate(["/", 7, 2], Environment()))
assert_equals(6, evaluate(["*", 2, 3], Environment()))
assert_equals(1, evaluate(["mod", 7, 2], Environment()))
assert_equals(True, evaluate([">", 7, 2], Environment()))
assert_equals(False, evaluate([">", 2, 7], Environment()))
assert_equals(False, evaluate([">", 7, 7], Environment()))
def test_defining_lambda_with_error_in_body():
"""The function body should not be evaluated when the lambda is defined.
The call to `lambda` should return a function closure holding, among other things
the function body. The body should not be evaluated before the function is called."""
ast = parse("""
(lambda (x y)
(function body ((that) would never) work))
""")
assert_is_instance(evaluate(ast, Environment()), Closure)
def test_call_to_function_should_evaluate_arguments():
"""Call to function should evaluate all arguments.
When a function is applied, the arguments should be evaluated before being bound
to the parameter names."""
env = Environment()
closure = evaluate(parse("(lambda (a) (+ a 5))"), env)
ast = [closure, parse("(if #f 0 (+ 10 10))")]
assert_equals(25, evaluate(ast, env))