[Haskell-cafe] Re: Partial Application

Benjamin Franksen benjamin.franksen at bessy.de
Thu Jan 18 16:29:47 EST 2007

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

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


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