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. > Please rewrite it as a regular > Python expression, preferably without looking at the patch that > has been proposed first. I bet there is a 50% chance that you get > it wrong (because there are two possible interpretations). But surely only one of them will agree with the standard definition of function composition. Both Mathworld and Wikipedia agree that f∘g(x) is equivalent to f(g(x)): http://mathworld.wolfram.com/Composition.html http://en.wikipedia.org/wiki/Function_composition and I don't see any reason why a compose() function shouldn't do the same. (Aside: how do I look at the patch? The only link I have is here: http://mail.python.org/pipermail/patches/2007-February/021687.html but I can't see how to get to the patch from there.) foo could be written as: def foo(data): return lambda *args, **kwargs: a(b(data)(c(*args, **kwargs))) Or without lambda: def foo(data): def composed(*args, **kwargs): return a(b(data)(c(*args, **kwargs))) return composed This soon gets unwieldy: def foo(arg1, arg2, arg3): return compose( f, g, h, factory(arg1), factory(arg2), factory(arg3) ) versus def foo(arg1, arg2, arg3): return lambda *a, **kw: ( f(g(h(factory(arg1)(factory(arg2)(factory(arg3)(*a, **kw)))))) ) but presumably composing six functions is rare. A further advantage of compose() is that one could, if desired, generate a sensible name and doc string for the returned function. Depends on how heavyweight you want compose() to become. I think the compose() version is far more readable and understandable, but another factor is the performance cost of the generated function compared to a hand-made lambda. For the record, Haskell makes compose a built-in operator: http://www.haskell.org/haskellwiki/Function_composition It doesn't appear to be standard in Ruby, but it seems to be commonly requested, and a version is on Facets: http://facets.rubyforge.org/apidoc/api/core/classes/Proc.html#M000161 -- Steven D'Aprano _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
