[Haskell-cafe] Writing functions in Haskell.

[Greg Buchholz]

Daniel Carrera wrote:
> Alright, I have what I believe must be a simple question. As one of the 
> exercises for the Haskell tutorial I got I have to implement an 
> alternative to the 'map' function.
> 
> This is what I have:
> 
> -----------------------------
> my/prompt $ cat Test.hs
> module Test
>     where
> 
> my_map p [] = []
> my_map p (x:xs) = p x : my_map p xs
> my/prompt $ hugs
> 
> [snip]
> 
> Hugs.Base> :l Test
> Test> :also Char
> Char> map toUpper "Hello"
> "HELLO"
> Char> my_map toUpper "Hello"
> ERROR - Undefined variable "my_map"

    Hugs can't find your version of "my_map" because it is sitting in
the "Char" module (notice the Char> prompt vs. Test>).  You could fully
qualify the function name (Test.my_map toUpper "Hello"), or you might
want to add "import Char" to your source file.  Something like...

$ cat test.hs

module Test where 
import Char

my_map p [] = []
my_map p (x:xs) = p x : my_map p xs

$ hugs      
__   __ __  __  ____   ___
||   || ||  || ||  || ||__      Hugs 98: Based on the Haskell 98
||___|| ||__|| ||__||  __||     Copyright (c) 1994-2003
||---||         ___||           World Wide Web: http://haskell.org/hugs
||   ||                         Report bugs to: hugs-bugs at haskell.org
||   || Version: November 2003
_________________________________________

Haskell 98 mode: Restart with command line option -98 to enable
extensions

Type :? for help
Prelude> :l test.hs
Test> my_map toUpper "Hello"
"HELLO"
Test>