Re: [GHC] #9781: Make list monad operations fuse

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 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 ):

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 _______________________________________________ ghc-tickets mailing list ghc-tickets@haskell.org http://www.haskell.org/mailman/listinfo/ghc-tickets