[GHC] #9781: Make list monad operations fuse

[Kim-Ee Yeoh]

>From the patch fragment at

https://phabricator.haskell.org/D455?id=1311#inline-3123

What's the justification for expanding out the definition of mapM from
"sequence . map f" into do-notation and duplicated code?

Observe how mapM now duplicates code from sequence.

The absence of benchmarks is bad enough. What's worse is that the given
excuse boils down to "Pity the poor compiler! Let's take over its work
instead. Like, just in case, you know."

This optimization work needs to take place at a higher level, with deep
understanding of existing compiler transformations. Otherwise, it's all a
code-muddying crapshoot in the dark.

-- Kim-Ee

On Tue, Nov 11, 2014 at 2:22 PM, GHC <ghc-devs at haskell.org> wrote:

> #9781: Make list monad operations fuse
> -------------------------------------+-------------------------------------
>               Reporter:  dfeuer      |            Owner:  dfeuer
>                   Type:  task        |           Status:  patch
>               Priority:  normal      |        Milestone:  7.10.1
>              Component:  Core        |          Version:  7.9
>   Libraries                          |         Keywords:
>             Resolution:              |     Architecture:  Unknown/Multiple
>       Operating System:              |       Difficulty:  Unknown
>   Unknown/Multiple                   |       Blocked By:
>        Type of failure:  Runtime     |  Related Tickets:
>   performance bug                    |
>              Test Case:              |
>               Blocking:              |
> Differential Revisions:  Phab:D455   |
> -------------------------------------+-------------------------------------
>
> Comment (by Herbert Valerio Riedel <hvr@…>):
>
>  In [changeset:"4923cea56345060faaf77e4c475eac6aa3c77506/ghc"]:
>  {{{
>  #!CommitTicketReference repository="ghc"
>  revision="4923cea56345060faaf77e4c475eac6aa3c77506"
>  Define list monad operations using comprehensions
>
>  Define list monad operations using list comprehensions. Code using monad
>  operations with lists did not fuse fully. Writing list code with `do`
>  notation or `(>>=)` and `(>>)` operations could allocate more than
>  equivalent code using list comprehensions.
>
>  Define `mapM` directly, instead of using `sequence` and `map`. This
>  leads to substantially less allocation in `cryptarithm2`.
>
>  Addresses #9781
>
>  Reviewed By: ekmett, nomeata
>
>  Differential Revision: https://phabricator.haskell.org/D455
>  }}}
>
> --
> Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9781#comment:4>
> GHC <http://www.haskell.org/ghc/>
> The Glasgow Haskell Compiler
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