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 Correio (MSN): malebria@riseup.net Jabber (GTalk): malebria@jabber.org Ekiga: malebria@ekiga.net IRC: malebria@irc.freenode.net malebria@irc.oftc.org Skype: marcotmarcot Telefone: 33346720 Celular: 98116720 Endereço: Rua Paula Cândido, 257/201 Gutierrez 30430-260 Belo Horizonte/MG Brasil