Why does signature change after an assignment

Question

Playing around in ghci I got the following expression: unlines . map (\(a,b) -> show a ++ " " ++ show b)

Now when I check it via :t I get:

> :t  unlines . map (\(a,b) -> show a ++ " " ++ show b)
unlines . map (\(a,b) -> show a ++ " " ++ show b)
  :: (Show a, Show a1) => [(a, a1)] -> String

So exactly as expected. But now if I try to assign it to some name, I get a more specific signature than the original one:

> let f = unlines . map (\(a,b) -> show a ++ " " ++ show b)
> :t f
f :: [((), ())] -> String

Why does this happen?

See also: [1](http://stackoverflow.com/questions/7055146), [2](http://stackoverflow.com/questions/11439163), [3](http://stackoverflow.com/questions/9714697), [4](http://stackoverflow.com/questions/8434808), [5](http://stackoverflow.com/questions/7799345), [6](http://stackoverflow.com/questions/8262020), [7](http://stackoverflow.com/questions/8655900), [8](http://stackoverflow.com/questions/11003535). I'm honestly not sure which (if any) of these to mark this a duplicate of. — Daniel Wagner, Mar 16 '13 at 22:40

score 12 · Accepted Answer · edited May 23 '17 at 10:29

12

Because of the monomorphism restriction, definitions of the form x = ... (no parameters) are given a monomorphic (i.e. non-polymorphic) type, which usually involves some defaulting as mentioned in the other answer.

To prevent this from happening, either add a type signature to your definition, or disable the monomorphism restriction using :set -XNoMonomorphismRestriction. You can add this to your .ghci file to have it run automatically on startup until it gets disabled by default in GHCi in some future version.

edited May 23 '17 at 10:29

Community

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answered Mar 16 '13 at 16:38

hammar

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Or eta expand; i.e. `let f x = unlines . map (\(a,b) -> show a ++ " " ++ show b) $ x` – luqui Mar 16 '13 at 17:44

score 4 · Answer 2 · answered Mar 16 '13 at 16:14

Defaulting rules.

When you type stuff in GHCi, it attempts to apply default types. IIRC, for things with a Num constraint it picks Integer, for Fractional it picks Double, and for everything else it picks ().

If you write this in a Haskell source file and load it into GHCi, this doesn't happen (I believe).

I think you can also say something like default Int to change the defaulting rules on a per-module basis.

Why does signature change after an assignment

2 Answers2