dynamically generated runtime methods & reflection

Bruno Desthuilliers bruno.42.desthuilliers at wtf.websiteburo.oops.com
Fri Jun 15 18:58:11 CEST 2007


Alex Martelli a écrit :
> Bruno Desthuilliers <bruno.42.desthuilliers at wtf.websiteburo.oops.com>
> wrote:
> 
>> Josiah Carlson a écrit :
>> (snip)
>>> Well, the particular operation is typically called 'currying a 
>>> function', 
>> <pedantic>
>> it's not 'currying' but 'partial application'.
>>
>> Currying is somehow the reverse of partial : it's a way of building a
>> multiple-args function from single-args functions.
>> </pedantic>
> 
> Wikipedia says 
 >
>"currying or Schönfinkelisation[1] "

cf below...

> "is the technique of
> transforming a function that takes multiple arguments into a function
> that takes a single argument"

The definition commonly agreed upon (in the FP world at least) is that 
currying is the process that "build" ("emulate", whatever...) 
multiple-args functions in a context that only supports single-arg 
functions (ie: ML, Haskell), so that (using Python syntax):

   f(x, y, z)

would really be in fact (and under the hood):

   f(x)(y)(z)

where f(x) returns a function closing over(x) and taking y, itself 
returning a function closing over x and y and taking z...

Talking about this:
"""
currying (...) reduces multiple-argument
functions to single-argument functions only (Schoenfinkel,
1924)
"""
http://srfi.schemers.org/srfi-26/mail-archive/msg00015.html


So while *very closely* related to partial application, it's not exactly 
the same thing.

FWIW, you can also have a look here: 
http://www.python.org/dev/peps/pep-0309/#motivation

> -- and FWIW I agree with Wikipedia in this
> case; 

I don't - and I'm not the only one:

http://lambda-the-ultimate.org/node/2266
"""
I had mistakenly learned that curry was a form of generalized partial 
application from the paper : Function Currying in Scheme by Jeffrey A. 
Meunier
and the Wikipedia entry (I should have known better), however I was 
mildly reprimanded for making this novice mistake in a recent paper 
submission to ICFP
"""

> the reverse (going from single-arg to multiple-args)

Note that I didn't say "going from", but "building". The context is a 
functional language where there's *no* such thing as "multiple args" 
functions.

Re-reading, I can agree that my choice of words may have been a bit 
poor, specially wrt/ the word "reverse". What I meant here was that 
partial application is used in the context of multiple-args function to 
'build' a function taking n-x arguments from a function taking n 
arguments, while currying is used in the context of single-arg functions 
to "emulate" multiple-args functions.

> would be
> "uncurrying", though I don't think I've ever used that term myself.
> 
> functools.partial's name may be more precise (because it can, e.g., go
> from a function taking 3 arguments to one taking 2 -- not just from N
> down to 1)
 >
> but your 'pedantic' remark seems pedantically wrong:-).

I certainly won't pretend knowing more about CS that you do, but given 
the definition of currying in pep-0309 - which exactly matches the 
definition I first learned when toying with Haskell -, I maintain my 
pedantic remark until proven wrong - which BTW should not be overly 
difficult if it happened to be the case !-)

As a last word, the original version of pep-0309 was named "curry", and 
has been corrected since:

"""
It isn't function currying, but partial application. Hence the name is 
now proposed to be partial().
"""
http://www.python.org/dev/peps/pep-0309/#feedback-from-comp-lang-python-and-python-dev




More information about the Python-list mailing list