[Haskell-cafe] newbie q about types

[Jonathan Cast]

On 1 Feb 2008, at 10:30 PM, Logesh Pillay wrote:

> I have a list.  Each component is a list with 2 whole numbers.

First off, why?  In Haskell (unlike dynamically typed languages like  
Python or Perl), if you know you always have two elements, you want a  
pair (519432,525806), not a list [519432,525806].

>   I want to multiply the second number by the log of the first
> eg
> tail ([519432,525806]) * log (head [519432,525806]).

You mean [519432,525806] !! 1 * log ([519432,525806 !! 0)

or

fst (519432,525806) * log (snd (519432,525806)

or (best of all)

case (519432, 525806) of
   (x, y) -> x * log y

or (if you must have a list)

case [519432, 525806] of
   [x, y] -> x * log y

> I get the ffg error message:
>
> No instance for (Floating [t])

This is because

tail :: [t] -> [t]

It doesn't give you a single element; it gives you the whole list!

jcc