Sirs/Mesdames:
We are considering a course using Haskell. We have "Hugs 98: Based on the
haskell 98 standard Copyright 1994-2001,"
downloaded just a few months ago. (This is under Windows NT.) The following
puzzles us --
>From the prelude:
take 1 [ ] -- expected to return the empty list [ ], instead
gives error message:
ERROR - Cannot find "show" function for:
*** Expression : take 1 [ ]
*** Of type : [a]
>From a module of ours with definition:
-- select nth item from a list
select :: Eq a => Int -> [a] -> [a]
select n lis
| n <= 0 || n > length lis = error "Invalid position!"
| n == 1 = [head lis]
| otherwise = select (n-1) (tail lis)
when called thus:
select 0 [ ]
Hugs gives the error message:
ERROR - Unresolved overloading
*** Type : Eq a => [a]
*** Expression : select 0 [ ]
Surely Haskell should be able to just return [ ] in both cases.
Thanks ahead for your response.
Prof Ernest Paquet
Walsh University
2020 Easton St. N.W.
Canton, OH, 44720 (USA)
Phone voice: 330-490-7229
fax: 330-490-7165
