[Haskell-cafe] Type infer

[Simon Peyton-Jones]

This isn't a type-soundness bug; but it could be considered a user-interface sort of bug.  After all, it's caused users to be puzzled.  It arises really because it was convenient for the implementation.

Do go ahead and file it as a Trac bug if it tripped you up enough to be worth fixing.  A fix should not take long, but might be a bit fiddly.

Simon

| -----Original Message-----
| From: haskell-cafe-bounces at haskell.org [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of Marco
| Túlio Gontijo e Silva
| Sent: 25 January 2007 12:57
| To: haskell-cafe
| Subject: Re: [Haskell-cafe] Type infer
|
| Em Qua, 2007-01-24 às 20:36 -0500, Bryan Donlan escreveu:
| > Marco Túlio Gontijo e Silva wrote:
| > > Hello,
| > >
| > > I'm trying to define a partition__ function that is like
| > > Data.Set.partition, but use State Monad:
| > >
| > >> import Data.Set
| > >> import Control.Monad.State
| > >
| > >> partition__ f =
| > >>     do
| > >>     snapshot <- get
| > >>     let
| > >>         (firsts, rest) = Set.partition f snapshot
| > >>     put rest
| > >>     return firsts
| > >
| > > When I try to infer it's type in ghci I got:
| > >
| > > $ ghci
| > >    ___         ___ _
| > >   / _ \ /\  /\/ __(_)
| > >  / /_\// /_/ / /  | |      GHC Interactive, version 6.6, for Haskell 98.
| > > / /_\\/ __  / /___| |      http://www.haskell.org/ghc/
| > > \____/\/ /_/\____/|_|      Type :? for help.
| > >
| > > Loading package base ... linking ... done.
| > > Prelude> :load partition.hs
| > > [1 of 1] Compiling Main             ( partition.hs, interpreted )
| > > Ok, modules loaded: Main.
| > > *Main> :type partition__
| > > partition__ :: (MonadState (Set a) t, Ord a) => (a -> Bool) -> t (Set a)
| > >
| > > Ok, then I add
| > >
| > >> partition__ :: (MonadState (Set a) t, Ord a) => (a -> Bool) -> t (Set
| > > a)
| > >
| > > to the file and then:
| > >
| > > *Main> :reload
| > > [1 of 1] Compiling Main             ( partition.hs, interpreted )
| > >
| > > partition.hs:4:0:
| > >     Non type-variable argument in the constraint: MonadState (Set a) t
| > >     (Use -fglasgow-exts to permit this)
| > >     In the type signature for `partition__':
| > >       partition__ :: (MonadState (Set a) t, Ord a) =>
| > >                      (a -> Bool) -> t (Set a)
| > > Failed, modules loaded: none.
| > >
| > > Why do I need glasgow-exts to specify a type infered by GHCi without
| > > -fglasgow-exts?
| >
| > I'd imagine the check that you're using -fglasgow-exts is performed when
| > parsing type signatures from the parser. When you allow GHC to infer the
| > type, it's pulling that from Control.Monad.State, which was compiled
| > with -fglasgow-exts - it's simply not checking that all the types you
| > might infer from there are legal without -fglasgow-exts.
|
| Makes sense, but isn't it a bug? It shouldn't be able infer types that
| are not allowed without -fglasgow-exts, right?
|
| Thanks.
|
| --
| malebria
| Marco Túlio Gontijo e Silva
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