Re: [boost] painless currying

Subject: Re: [boost] painless currying
From: Eric Niebler (eric_at_[hidden])
Date: 2011-08-23 22:59:36

On 8/23/2011 5:21 PM, Mathias Gaunard wrote:
> On 08/23/2011 11:15 PM, Mathias Gaunard wrote:
>> On 08/23/2011 09:38 PM, Eric Niebler wrote:
>>> After playing around with functional languages, I've come to envy how
>>> easy they make it to curry functions. Call a 2-argument function with 1
>>> argument and you get a function that takes 1 argument. Pass another
>>> argument and it evaluates the function. Simple. In contrast, C++ users
>>> have to use binders, which are not as nice.
>>>
>>> On a lark, I implemented a wrapper that turns any TR1-style function
>>> object into a "curryable" function object (attached). Successive
>>> function call invocations bind arguments until enough arguments are
>>> bound to invoke the wrapped function. With it you can do the following:
>>>
>>> curryable<std::plus<int> > p;
>>> auto curried = p(1);
>>> int i = curried(2);
>>> assert(i == 3);
>>>
>>> Is there any interest in such a thing?
>>
>> It could be argued that Phoenix actors should be doing this.
>>
>> I also asked a lot of time ago for an easy function to turn a pfo into a
>> phoenix actor, which I think would make phoenix much more ubiquitous.
>>
>> assuming that function was called lazy, and Phoenix did currying
>> automatically, you could do
>>
>> int i = (lazy(std::plus<int>)(1) + 4)(3)
> 
> Sorry, this should have been
> int i = (lazy(std::plus<int>())(1) + 4)(3)
> of course.

Could you explain this? I'm guessing that lazy(std::plus<int>()) creates
a binary function, and that lazy(std::plus<int>())(1) binds the first
argument and returns a unary function. But what does it mean to add an
integer to a unary function? That doesn't make sense to me.

> And I remember why Phoenix doesn't do currying: that's because that only
> works with monomorphic function objects.

What only works with monomorphic functions?

> Detecting and propagating monomorphism could be nice though. It could
> eventually provide better error messages, faster compilation times, and
> automatic currying on monomorphic function objects.

What do monomorphic functions have to do with this? My currying code
works with polymorphic functions.

Sorry if I'm being dense. It's pretty late over here.

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com