Philippe de Rochambeau wrote:
my original query concerning partial application was triggered by the following statement from Thomson's "The Craft of Functional Programming", p. 185:
" multiplyUC :: (Int, Int) -> Int multiplyUC (x,y) = x * y
multiply :: Int -> Int -> Int multiply x y = x * y
....
In the case of multiplications we can write expression like multiply 2".
When I read this, I thought that you could partially apply "multiply" by typing "multiply 2" at the ghci prompt.
(This has already been said but to re-iterate:) You can (partially apply "multiply") but not by typing "multiply 2" at the ghci prompt. The latter is interpreted by ghci as a command to evaluate and then /print/ the resulting value, which means it must convert it to a textual representation, using the Show class, which is normally not instantiated for function values.
However, this generated an error:
<interactive>:1:0: No instance for (Show (Int -> Int)) arising from use of `print' at <interactive>:1:0-9 Possible fix: add an instance declaration for (Show (Int -> Int)) In the expression: print it In a 'do' expression: print it
Just import (:load) the module Text.Show.Functions which defines a Show instance for functions. Prelude> :m Text.Show.Functions Prelude Text.Show.Functions> let multiply x y = x * y Prelude Text.Show.Functions> multiply 2 <function> Or, use let to bind the result to a variable, like Prelude> let multiply x y = x * y Prelude> let f = multiply 2 Prelude> f 3 6 Cheers Ben