On 1/28/06, Andrew Savige
--- Brian Hulley wrote:
def myeval(x, y, op) x.send op, y end
This could be done in Haskell by
myeval :: a->b->(a -> b -> c) -> c myeval x y op = op x y
eg myeval (+) 1 2
Though the following program does indeed work:
myeval :: (Int -> Int -> Int) -> Int -> Int -> Int myeval op x y = op x y
main :: IO () main = do putStrLn $ show (myeval (+) 2 3)
I can't hardwire (+) because the operator is unknown at compile time; it is input to the program at run time as a string ("+" in this example).
My problem is how to convert the string "+" to the operator (+) at run time. Well, I could do it with a lookup table:
mycvt :: String -> (Int -> Int -> Int) mycvt "+" = (+) mycvt "-" = (-) mycvt "*" = (*)
but that is what I'm trying to avoid (I was naively hoping that the read function could magically do it somehow :-).
It's probably a bit more heavy-weight than what you were looking for, but take a look at: http://www.cse.unsw.edu.au/~dons/hs-plugins/ It allows you to load and execute Haskell code at run-time. I suppose this should also be possible by importing some part of GHC (perhaps GHCi). /S -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862