An idea for a different style of metaprogramming evaluation using the optimiser

[Joachim Breitner]

> In my hacked together implementation of this, constant folding happens
too late so only the algebraic version teminated.

ah, very good point!

Joachim 



Am 28. Februar 2018 06:26:35 EST schrieb Matthew Pickering <matthewtpickering at gmail.com>:
>You have identified a small inaccuracy in my original email.
>
>It is true that ultimately 3 == 0 will be evaluated but these constant
>folding opportunities are only applied later after other inlining
>takes place.
>
>It becomes quite crucial under this scheme when different
>optimisations happen because when we inline a recursive function, we
>have to make sure to apply the right evaluation steps before trying to
>simplify the function body.
>In my hacked together implementation of this, constant folding happens
>too late so only the algebraic version teminated.
>
>Cheers,
>
>Matt
>
>On Tue, Feb 27, 2018 at 5:01 PM, Joachim Breitner
><mail at joachim-breitner.de> wrote:
>> Hi,
>>
>> something like this would be great. I don’t have a sense yet of what
>> “something” should be like.
>>
>>
>> Am Dienstag, den 27.02.2018, 09:59 +0000 schrieb Matthew Pickering:
>>> To go back to the power example, the recursive
>>> condition would have to be an inductively defined natural (data N =
>Z
>>> | S N) rather than an Int as the comparison operator for integers
>>> can't be evaluated by the optimiser.
>>
>> Sure they can:
>>
>> $ cat ConstantFolding.hs
>> {-# LANGUAGE TemplateHaskell #-}
>> {-# OPTIONS_GHC -fplugin=Test.Inspection.Plugin #-}
>> module ConstantFolding where
>>
>> import Test.Inspection
>>
>> ltInt :: Bool
>> ltInt = (3::Int) > 2
>>
>> ltInteger :: Bool
>> ltInteger = (3::Integer) > 2
>>
>> true :: Bool
>> true = True
>>
>>
>> inspect $ 'ltInt === 'true
>> inspect $ 'ltInteger === 'true
>>
>> $ ghc -O ConstantFolding.hs
>> [1 of 1] Compiling ConstantFolding  ( ConstantFolding.hs,
>> ConstantFolding.o )
>> ConstantFolding.hs:17:1: ltInt === true passed.
>> ConstantFolding.hs:18:1: ltInteger === true passed.
>> inspection testing successful
>>       expected successes: 2
>>
>>
>>
>> As  an alternative with a maybe simpler user interface (and probably
>> less power), I wonder if we can create a magic function
>>> compileTimeWHNF :: a -> a
>> or
>>> compileTimeNF :: a -> a
>> and a GHC core plugin (or eventually built-in thing) that finds these
>> magic functions and evaluates their arguments, using the simplifier.
>>
>>
>> Cheers,
>> Joachim
>>
>> --
>> Joachim Breitner
>>   mail at joachim-breitner.de
>>   http://www.joachim-breitner.de/