Hi Oscar,
Define
result = (.)
Then '(result f) g' applies f to the result part of a function g. That's
what you want when negating the result of not. For (==), you want to negate
the result of the result, so you'd instead say '(result.result) not (==)'.
Keep composing result for deeper applications.
If you want to edit/transform the first or second part of a pair instead of
the result of a function, then you'd use 'first' or 'second' in place of
'result'.
If you think of f (here f=not) as an "semantic editor" (transformer) of
values, then 'result', 'first', and 'second' are "semantic editor
combinators", which can be composed together arbitrarily. See
http://conal.net/blog/semantic-editor-combinators .
- Conal
2008/12/26 Oscar Picasso
Hi,
I can write: *Main> let yes = not . not *Main> :t yes yes :: Bool -> Bool
But not: *Main> let isNotEqual = not . (==)
<interactive>:1:23: Couldn't match expected type `Bool' against inferred type `a -> Bool' Probable cause: `==' is applied to too few arguments In the second argument of `(.)', namely `(==)' In the expression: not . (==)
Why?
Oscar