Function Composition

[Jed Wesley-Smith]

(forgot to post to the list)

The reason f compose g is "natural" is that you get x -> f(g(x))

If you want the alternate f andThen g then you get x -> g(f(x))

As I gave as reference in the original email:

http://en.wikipedia.org/wiki/Function_composition_(computer_science)

as well as the usual math definition:

http://en.wikipedia.org/wiki/Function_composition

for further reference, here's rosettacode:

http://rosettacode.org/wiki/Function_composition

Yes, Scala does use compose/andThen – this is likely to be a specific
and particular source of confusion and issues as it is a particularly
likely place Java programmers will have contact with the idiom.

This notion of compose is very well established.

cheers,
jed.

On 2 January 2013 11:29, John Nilsson <john at milsson.nu> wrote:
> Btw, compose and andThen feels like Scala to me. So there's some precedence
> for that choice of names. Did you have any other source in mind? Besides the
> proliferation of the ∘ in math texts I can't really think of any specific
> common understanding of "compose" meaning some specific thing, I don't think
> Scala qualifies as "common" at this time.
> So my argument was that even the math people can agree to interpret
> "compose" as ";", while less commonly used, it's the same operation.
> Given that we both agree that "andThen" should be used instead, why even
> have the other? And if there is only the one, why not call it by it's
> natural name "compose"?
>
> BR,
> John
>
>
> On Wed, Jan 2, 2013 at 1:17 AM, Jed Wesley-Smith <jed at wesleysmith.io> wrote:
>>
>> Well, I'm not arguing the merits of one or the other, indeed I'd like
>> both (and I usually use the andThen/; form). I'd just argue that this
>> is a well established name/form and there is value (as in least
>> surprise) in following the precedent.
>>
>> Function<A, B> f = …
>> Function<B, C> g = …
>>
>> Function<A, C> c = g.compose(f);
>> Function<A, C> d = f.andThen(g);
>>
>> I'd argue the second form reads better anyway!
>>
>> cheers,
>> jed.
>>
>> On 2 January 2013 11:07, John Nilsson <john at milsson.nu> wrote:
>> > For what it's worth. I think the math people got it wrong with "∘". The
>> > other operation they call compose: ";" - which has applicative order -
>> > feels
>> > more natural. So one vote for keeping compose as ";". Also, this is the
>> > composition operation for statements Java programmers have been using
>> > for
>> > years with no complaint, so why arbitrarily switch to "∘" for function
>> > composition?
>> > Besides even the math people revert to applicative order when modelling
>> > function composition as a category.
>> >
>> > BR,
>> > John
>> >
>> >
>> > On Wed, Jan 2, 2013 at 12:40 AM, Jed Wesley-Smith <jed at wesleysmith.io>
>> > wrote:
>> >>
>> >> hello lambdarians!
>> >>
>> >> Just noticed that the java.util.function.Function.compose method is
>> >> round the wrong way (according to the common definition of function
>> >> composition*).
>> >>
>> >> The current lambda definition:
>> >>
>> >>
>> >>
>> >> http://hg.openjdk.java.net/lambda/lambda/jdk/file/858e2f478d62/src/share/classes/java/util/function/Function.java
>> >>
>> >> has f.compose(g) mean apply f and then apply g, whereas the usual
>> >> (most other languages and the literature) has f ∘ g mean apply g and
>> >> then apply f.
>> >>
>> >> The form that Function has currently is commonly known as andThen. So,
>> >> you could rename the current method andThen and add a new compose
>> >> method that works in the usual manner.
>> >>
>> >> cheers,
>> >> jed.
>> >>
>> >> * references:
>> >> http://en.wikipedia.org/wiki/Function_composition
>> >> http://en.wikipedia.org/wiki/Function_composition_(computer_science)
>
>