[Python-Dev] functools.compose to chain functions together (original) (raw)

Xavier Morel xavier.morel at masklinn.net
Mon Aug 17 12:38:57 CEST 2009

On 17 Aug 2009, at 09:43 , Steven D'Aprano wrote:

On Mon, 17 Aug 2009 08:10:16 am Martin v. Löwis wrote:

I don't think he did. Comparing it to the one obvious solution (use a lambda expression), his only reasoning was "it is much easier to read". I truly cannot believe that a compose function would be easier to read to the average Python programmer: if you have

def foo(data): return compose(a, b(data), c) what would you expect that to mean? foo is a factory function that, given an argument data, generates a function b(data), then composes it with two other functions a and c, and returns the result, also a function. From his messages, I think Martin's issue with compose is with the
composition order rather than the fact that it "pipes" functions:
compose uses the mathematical order, (f ∘ g)(x) = f(g(x)) (so g, the
last function of the composition, is applied first), rather than a
"shell pipe" order of (f >>> g)(x) = g(f(x)) (where g, the last
function of the composition, is applied last).

For the record, Haskell makes compose a built-in operator:

http://www.haskell.org/haskellwiki/Functioncomposition

Yes, but Haskell also has a left-to-right composition, the (>>>)
operator: http://haskell.org/ghc/docs/latest/html/libraries/base/Control-Arrow.html#v :>>>

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