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-evaluati...
On Sun, Apr 15, 2012 at 22:31, Ting Lei
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
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