Dear all,
lately I typed in the following expressions ...
Main> []
ERROR: Cannot find "show" function for:
*** Expression : []
*** Of type : [a]
Main> reverse []
ERROR: Cannot find "show" function for:
*** Expression : reverse []
*** Of type : [a]
Main> error "test"
ERROR: Cannot find "show" function for:
*** Expression : error "test"
*** Of type : a
Each of the inputs is answered by an error message! Is this what we
would expect? Can you see why? As another example, consider the
following simple program.
> data Tree a = Empty
> | Node (Tree a) a (Tree a)
> hd Empty = error "hd: empty tree"
> hd (Node Empty a r) = a
> hd (Node l@(Node _ _ _) a r) = hd l
> tl Empty = error "tl: empty tree"
> tl (Node Empty a r) = r
> tl (Node l@(Node _ _ _) a r) = Node (tl l) a r
As a start we type in some simple test cases
Main> hd (Node Empty 2 Empty)
2
Main> hd Empty
ERROR: Cannot find "show" function for:
*** Expression : hd Empty
*** Of type : a
Main> tl (Node Empty 2 Empty)
ERROR: Cannot find "show" function for:
*** Expression : tl (Node Empty 2 Empty)
*** Of type : Tree Integer
Main> tl Empty
ERROR: Cannot find "show" function for:
*** Expression : tl Empty
*** Of type : Tree a
Yes, of course: the `deriving (Show)' clause is missing.
> data Tree a = Empty
> | Node (Tree a) a (Tree a)
> deriving (Show)
We try again, only to find that somehow the base cases are not handled
properly.
Main> hd (Node Empty 2 Empty)
2
Main> hd Empty
ERROR: Cannot find "show" function for:
*** Expression : hd Empty
*** Of type : a
Main> tl (Node Empty 2 Empty)
Empty
Main> tl Empty
ERROR: Cannot find "show" function for:
*** Expression : tl Empty
*** Of type : Tree a
The solution to the problem is that Hugs refuses to print values of
polymorphic type! We have
[] :: [a]
reverse [] :: [a]
error "test" :: a
hd Empty :: a
tl Empty :: Tree a
In each case the type involves a type variable.
Could Hugs be a bit more clever here? Good old Miranda distinguished
between scripts and command-level expressions. In scripts this
restriction is sensible, on the top-level it is not. So please, change
Hug's behaviour which is certainly a miracle to beginners and maybe
also to the experienced Haskell programmer.
Cheers, Ralf
