Another appeal to fix Function.compose

[Stephen Colebourne]

This change means there is no way to do the original behaviour. As a
non-functional Java programmer, I would read f.compse(g) as a
"function that does f, then does g", ie. as per the original
behaviour. The new compose behaviour seems entirely backwards (again,
as a non FP programmer).

I don't mind this change, so long as there is an equivalent that does
the original behaviour that I would see as more natural (again, as a
non FP programmer).

Stephen


On 28 March 2013 00:24, Mike Duigou <mike.duigou at oracle.com> wrote:
> Fixed.
>
> http://hg.openjdk.java.net/lambda/lambda/jdk/rev/fa66ee11db44
>
> Sorry for the delay. There was no great debate just many, many fish to be fried.
>
> To those already using Function.compose()--we apologize in advance for breaking your code with this change.
>
> Mike
>
> On Mar 26 2013, at 04:52 , Jed Wesley-Smith wrote:
>
>> Previously I have appealed to the lambda-dev community to fix the
>> semantics of the Function::compose method to fall into line with all
>> other languages and mathematic's general understanding of function
>> composition.
>>
>> In most languages, and in most maths texts, function composition is
>> presented as:
>>
>> f . g
>>
>> or
>>
>> f composed with g
>>
>> where function composition is the application of one function to the
>> results of another.
>>
>> In other words, the semantics of composition of f and g is:
>>
>> f(g(_)) or: f applied to the result of g applied to the argument.
>>
>> As it currently stands, f.compose(g)* has the opposite semantics; it
>> says that the f will be applied first _and then_ g will be applied,
>> ie:
>>
>> g(f(_)) or: g applied to the result of f applied to the argument.
>>
>> There is a perfectly good name for this combinator: andThen.
>>
>> While I totally understand why to the OO developer "f compose g" may
>> feel like g(f(_)) or f.andThen(g), there is a significant prior work
>> that defines it the other way. To go against that is merely arbitrary
>> and contrariwise.
>>
>> So, my belated appeal is to fall in line with the standard definition
>> of compose, or to rename the current combinator andThen.
>>
>> cheers,
>> jed.
>>
>> * http://hg.openjdk.java.net/lambda/lambda/jdk/file/112b2a64495a/src/share/classes/java/util/function/Function.java
>>
>
>