compose

[Eric Brunel]

xam wrote:
> i've been truing to get this 'simple' code to work,
> def compose(*funcs):
> ...  if len(funcs)>1:
> ...    return lambda x:funcs[0](compose(funcs[1:])(x))
> ...  else:  return lambda x:funcs[0](x)
> 
>>>>compose(add_5, mul_3, sub_2)(2)
>>>
> Traceback (most recent call last):
>   File "<interactive input>", line 1, in ?
>   File "<interactive input>", line 3, in <lambda>
>   File "<interactive input>", line 4, in <lambda>
> TypeError: 'tuple' object is not callable
> does anyone want to give it a shot?

Let me guess: have you done any Lisp before? ;-)

Try it like this:

def compose(*funcs):
   if len(funcs)>1:
     return lambda x:funcs[0](compose(*funcs[1:])(x))
   else:
     return lambda x:funcs[0](x)

If you don't put the '*' in front of the funcs[1:] in the first recursive call, 
you call compose with a single argument which is a tuple. Another way would be 
to write:

def compose(*funcs):
   if len(funcs)>1:
     return lambda x:funcs[0](apply(compose, funcs[1:])(x))
   else:
     return lambda x:funcs[0](x)

Try to print funcs at the beginning of compose with both versions to see what I 
mean.

HTH
-- 
- Eric Brunel <eric.brunel at pragmadev.com> -
PragmaDev : Real Time Software Development Tools - http://www.pragmadev.com