[Haskell] nwebie question
sebastian.sylvan at gmail.com
Sat Oct 1 15:13:07 EDT 2005
On 10/1/05, justebelmont at argentina.com <justebelmont at argentina.com> wrote:
> Hi there folks, I'm trying to learn some functional programming in
> well, at the College, but they don't teach anything, lol, so I have a
> simple question about type creation.
> This is a new type that contains the int and the new infinity number
> data NatInf = Infinity | Num Int
> At this point every seems to be ok, but when I load the file in Hugs,
> then write
> Num 5 (or Infinity)
> the following error appears:
> ERROR – Cannot find "show" function for:
> *** Expression : Num 5
> *** Of type: : NatInf
This means that you have not yet specified how you want your new data
type to be converted to a string (using the function "show" in the
Show type class).
Either you write your own implementation
instance Show NatInf where
show (Num x) = ...
show (Infinity) = ...
Or you can simply derive a standard one by typing "deriving Show" at
the end of your data declaration.
data NatInf = Infinity | Num Int deriving Show
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