[Haskell-cafe] strange GHCi type inference behavior involving map and partially applied functions

[Ting Lei]

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: hesselink at 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: tinlyx at hotmail.com
> CC: haskell-cafe at 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 <tinlyx at 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 at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
 		 	   		  
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20120415/dbc34d92/attachment.htm>