Eric: Thanks a lot for explaining the issue. That's very helpful. Kind of a newbie question, how can I automatically turn on NoMonomorphismRestriction using emacs mode for haskell? I know there is the :set command, but I don't want to do it every time I start up emacs. In my humble opinion, I think the GHC should turn on ExtendedDefaulting if GHCi has it on by default. Otherwise it is confusing for newbies. Also, the defaulting is a kind of wierd and arbitrary behavior as I see it. In the case I showed, it gives unintuivive (or even errorneous) behavior. A function that can be applied now suddenly causes type mismatches. Isn't it a good thing in Haskell that if you get the type right, you get a big portition your program right? If turning on ExtendedDefaulting causes too much trouble, maybe then GHC should turn it off by default in both GHI and GHCi. In either case, I think the two should be consistent. Thanks again for help, Ting
> From: hessel...@gmail.com > Date: Sun, 15 Apr 2012 22:45:47 +0200 > Subject: Re: [Haskell-cafe] strange GHCi type inference behavior involving > map and partially applied functions > To: tin...@hotmail.com > CC: haskell-cafe@haskell.org > > GHCi is defaulting the 'a' in 'Show a' to unit because of the extended > defaulting feature [1] in GHCi. If you turn on > NoMonomorphismRestriction in GHCi, you get the same behavior as in > GHC. If you turn on ExtendedDefaulting in GHC, you get the same > behavior as in GHCi. > > Erik > > [1] > http://www.haskell.org/ghc/docs/latest/html/users_guide/interactive-evaluation.html#extended-default-rules > > On Sun, Apr 15, 2012 at 22:31, Ting Lei <tin...@hotmail.com> wrote: > > Hi All, > > > > I found a really strange case where GHC and GHCi behave differently in > > inferring types. It took me hours to figure this out. > > > > The following program > > > > {-# LANGUAGE NoMonomorphismRestriction #-} > > g x i = x ++ show i > > [a,b] = map g ["X","Y"] > > > > will not load without "NoMonomorphismRestriction". With that option, it > > will load and return the correct types (as expected): > > *Main> :t [a,b] > > [a,b] :: Show a => [a -> [Char]] > > > > *Main> a 1 > > "X1" > > > > However, if I do the same thing GHCi, the type inferencing seems to have > > been messed up: > > > > *Main> let g x i = x ++ show i > > *Main> let [a,b] = map g ["X","Y"] > > *Main> :t [a,b] > > [a,b] :: [() -> [Char]] > > *Main> :t map g ["X","Y"] > > map g ["X","Y"] :: Show a => [a -> [Char]] > > > > Note how in the last two instances the terms on the left and right-hand > > sides of the definition return different types. Also, the correct return > > type should be a list of unary functions taking an (Show a) as > > the parameter. Now the result is unusable: > > > > *Main> a 1 > > <interactive>:52:3: > > No instance for (Num ()) > > arising from the literal `1' > > Possible fix: add an instance declaration for (Num ()) > > In the first argument of `a', namely `1' > > In the expression: a 1 > > In an equation for `it': it = a 1 > > > > I am using GHCi 7.4.1 under windows. I also tried this under GHC 7.0x > > Is this a GHCi bug or could anyone please explain why this can of strange > > behavior happens? > > If this is a bug, could anyone with an account help file a bug for this? > > > > Thanks in advance, > > > > Ting > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe@haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
