Hi all,

trying to solve the problem 6 "reverse a list"

I'm getting a different behaviour from ghci / runhaskell.

main = do
putStrLn $ concat (map show (myReverse [1,2,3]))
-- putStrLn $ concat (map show (myReverse []))

myReverse :: [a] -> [a]
myReverse [] = []

myReverse (x:xs) = go [] (x:xs)
where go :: [a] -> [a] -> [a]
go curr (x:xs) | null xs = [x] ++ curr

go curr (x:xs) = go ([x] ++ curr) xs

the second putStrLn (once uncommented) gives this error in the runhaskell/ghc:

Ambiguous type variable `a0' in the constraint:

(Show a0) arising from a use of `show'

Probable fix: add a type signature that fixes these type variable(s)

In the first argument of `map', namely `show'

_but_ if I load the same source into ghci I can successfully enter&run the command:

Prelude> :reload
[1 of 1] Compiling Main ( 05.hs, interpreted )
Ok, modules loaded: Main.

*Main> putStrLn $ concat (map show (myReverse []))

*Main>

Then if I "help" the type inference changing the myReverse type declaration to:

myReverse :: [Int] -> [Int]

everything works well

So, my question is: _where_ is the difference between ghci / runhaskell?

thx