haskell types ghci monomorphism-restriction

ghci - defaulting confusion

I happened to see some strange behaviour during checking the size (minBound,maxBound) and "length in decimal representation" of different integral types.

Using GHCi:

Prelude> :{
Prelude| let mi = minBound
Prelude|     ma = maxBound
Prelude|     le = fromIntegral $ length $ show ma
Prelude|  in [mi,ma,le] :: [Int]
Prelude| :}
[-9223372036854775808,922372036854775807,2]
                                         ^

in the last place I would expect 19.

My first guess is that maxBound defaults to () and thus yields 2, but I don't understand that because ma should be an Int by the explicit type annotation (:: [Int]) - and by referential transparency all symbols named ma should be equal.

If I put the statement above in a file and load it into GHCi, I get the correct result.

So why do I get a wrong result?

Solution

Confusingly, this is still the monomorphism restriction at play (or rather the lack thereof when in GHCi). Since GHCi doesn't have the monomorphism restriction enabled, your definitions of mi and ma don't get specialized to Int as you think they will - instead they stay general as mi, ma :: Bounded a => a and the a variable gets instantiated twice

once as () in fromIntegral $ length $ show ma (as you observed, this is a default)
once as Int in [mi,ma,le] :: [Int]

If you want mi and ma to actually be of type Int, annotate them as such directly

Prelude> :{
Prelude| let mi, ma :: Int
Prelude|     mi = minBound
Prelude|     ma = maxBound
Prelude|     le = fromIntegral $ length $ show ma
Prelude|  in [mi,ma,le]
Prelude| :}
[-9223372036854775808,9223372036854775807,19]

Or turn on the monormorphism restriction manually in GHCi

Prelude> :set -XMonomorphismRestriction
Prelude> :{
Prelude| let mi = minBound
Prelude|     ma = maxBound
Prelude|     le = fromIntegral $ length $ show ma
Prelude| in [mi,ma,le] :: [Int]
Prelude| :}
[-9223372036854775808,9223372036854775807,19]