Re: In a Pascal-like language, would being able to declare a subroutine as "purely functional" be of any value?

On Fri, Mar 11, 2011 at 07:55:22PM +0000, glen herrmannsfeldt wrote:
> noitalmost <noitalmost@cox.net> wrote:
>
> > I was thinking of something like:
> > pure function foo(x,y : int) int
> > begin
> > ...
> > end;
>
> Fortran has PURE. You might look at how it was done.
>
> In addition, variables aren't allowed to have the SAVE attribute
> (static in some other languages). ...


In Haskell, where functions are pure by default, there is an escape
hatch called unsafePerformIO so that impure code can be smuggled into
otherwise pure functions without upsetting the type checker.


Using unsafePerformIO, you can perform logging, e.g. like this:


import System.IO
import System.IO.Unsafe


add a b = unsafePerformIO $ do
        putStrLn $ "add " ++ (show a) ++ " + " ++ (show b)
        return $ a + b


The type of add here is (Num a) => a -> a -> a (i.e. it takes two instances
of Num and returns an instance of Num). This type is the same as if we had
written:


add a b = a + b


which is a pure function.


Regards,


Bob

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