This document describes functional pattern matching in different programming languages like Haskell, Erlang, Python, and Prolog. It introduces a Python library called Patterns that implements pattern matching via function decoration and AST transformation. The library allows matching different patterns in function arguments and assigning or evaluating expressions based on the matched patterns.

1. FUNCTIONAL
PATTERN
MATCHING
DAKER PINHEIRO

2. DAKER FERNANDES
PINHEIRO
UFPE
INDT RECIFE
WEBKIT (NIX)
QT, KDE, ...
C++, C, PYTHON, JAVASCRIPT, PROLOG, ...
PYTHON SINCE 2009

3. PROLOG
botname("Chatty").
chatterbot(['hello'], ['Hello', ',', 'my', 'name', 'is', B]) :-
botname(B).
chatterbot(['how', 'much', 'is' | Expr], ['It', 'is'], Result) :-
eval_expression(Expr, Result).
chatterbot(['my', 'name', 'is', Name], ['Nice', 'to', 'meet', 'you']) :-
assert(name(Name)).

4. PATTERN
MATCHING

5. HASKELL
fib :: Int -> Int
fib 1 = 1;
fib n = n * fib n - 1;

6. HASKELL
count :: [n] -> n -> n
count (n:tail) n = 1 + count tail n;
count (k:tail) n = count tail n;
count [] _ = 0;

7. ERLANG
greet(male, Name) ->
io:format("Hello, Mr. ~s!", [Name]);
greet(female, Name) ->
io:format("Hello, Mrs. ~s!", [Name]);
greet(_, Name) ->
io:format("Hello, ~s!", [Name]).

8. PYTHON!!
@patterns
def fib():
if 1: 1
if n: n * fib(n - 1)

9. DISCLAIMER

10. PIPINSTALLPATTERNS
... OR CLONE IT.

11. HOW IT WORKS?
@patterns
if function():
if RULE1: EXPRESSION1
if RULE2: EXPRESSION2
...
if function(*args):
if match(args, RULE1):
ASSIGNS1
return EXPRESSION1
elif match(args, RULE2):
ASSIGNS2
return EXPRESSION2
else:
raise Mismatch()

12. PYTHON
@patterns
def fib():
if 1: 1
if n: n * fib(n - 1)
def fib(n):
if n == 1:
return 1
else:
return n * fib(n - 1)

13. TREE
@patterns
def depth():
if ('Tree', _, left, right): 1 + max(depth(left), depth(right))
if None: 0
def depth(tree):
if tree:
return max(depth(tree[2]), depth(tree[3]))
else:
return 0

14. CHATTERBOT
botname = "Chatty"
@patterns
def answer():
if ['hello']:
"Hello, my name is %s" % botname
if ['hello', 'my', 'name', 'is', name]:
"Hello, %s!" % name.capitalize()
if ['how', 'much', 'is'] + expr:
"It is %d" % eval(' '.join(expr))
if ['bye']:
"Good bye!"

15. CHATTERBOT
botname = "Chatty"
def answer(message):
if message == ['hello']:
return "Hello, my name is %s" % botname
elif message[:-1] == ['hello', 'my', 'name', 'is']:
return "Hello, %s" % message[-1].capitalize()
elif message[:3] == ['how', 'much', 'is']:
return "It is %d" % eval(' '.join(expr))
elif message == ['bye']:
return "Good bye!"
else:
raise Mismatch()

16. BREATH DEEP,
SEEK PEACE

17. PATTERNS
import sys, inspect, ast, re
from ast import *
def patterns(func):
empty_argspec = inspect.ArgSpec(args=[], varargs=None,
keywords=None, defaults=None)
assert inspect.getargspec(func) == empty_argspec
# TODO: make it not as weird and dirty
func.__globals__['Mismatch'] = Mismatch
tree = get_ast(func)
transform_function(tree.body[0])
return compile_func(func, tree)

18. GET_AST
def get_ast(func):
# Get function source
source = inspect.getsource(func)
# Fix extra indent if present
spaces = re_find(r'^s+', source)
if spaces:
source = re.sub(r'(^|n)' + spaces, 'n', source)
return ast.parse(source, func_file(func), 'single')

19. TRANSFORM_FUNCTION
def transform_function(func_tree):
assert all(isinstance(t, If) for t in func_tree.body),
'Patterns function should only have if statements'
# Adjust arglist and decorators
func_tree.args.args.append(Name(ctx=Param(), id='value'))
func_tree.decorator_list = []
...

20. TRANSFORM_FUNCTION
...
for test in func_tree.body:
cond = test.test
...

21. TRANSFORM_FUNCTION
...
if isinstance(cond, (Num, Str, List, Tuple)) and not has_vars(cond):
test.test = make_eq(N('value'), cond)
if isinstance(cond, (Num, Str, Name, Compare, List,
Tuple, Dict, BinOp)):
tests, assigns = destruct_to_tests_and_assigns(N('value'), cond)
test.test = BoolOp(op=And(), values=tests) if tests else V(1)
test.body = assigns + test.body
else:
raise TypeError("Don't know how to match %s" % ...)
...

22. TRANSFORM_FUNCTION
...
if isinstance(cond, (Num, Str, List, Tuple)) and not has_vars(cond):
test.test = make_eq(N('value'), cond)
if isinstance(cond, (Num, Str, Name, Compare, List,
Tuple, Dict, BinOp)):
tests, assigns = destruct_to_tests_and_assigns(N('value'), cond)
test.test = BoolOp(op=And(), values=tests) if tests else V(1)
test.body = assigns + test.body
else:
raise TypeError("Don't know how to match %s" % ...)
...

23. TRANSFORM_FUNCTION
...
func_tree.body = map(wrap_tail_expr, func_tree.body)
func_tree.body.append(
Raise(type=N('Mismatch'), inst=None, tback=None))

24. TESTS_AND_ASSIGNS
def destruct_to_tests_and_assigns(topic, pattern):
if isinstance(pattern, (Num, Str)):
return [make_eq(topic, pattern)], []
elif isinstance(pattern, Name):
return [], [make_assign(pattern.id, topic)]
elif isinstance(pattern, Compare) and
len(pattern.ops) == 1 and isinstance(pattern.ops[0], Is):
return [make_call('isinstance', topic, pattern.comparators[0])],
[make_assign(pattern.left.id, topic)]
...

25. A NEW PEP? :-)

26. Q & A
DAKER FERNANDES PINHEIRO
HTTP://CODECEREAL.BLOGSPOT.COM