[Haskell-beginners] Choosing a function by (string) name...

[Stuart Hungerford]

Hi,

Suppose I have a module of functions with identical signatures:

module Functions where

a :: Int -> [Int]

a x = [x + 1, x + 2, x + 3]

b :: Int -> [Int]

b x = [x + 4, x + 5, x + 6]

and in my main function I receive a command line arguments (a string)
naming one of the functions in MyModule. I'd like to lookup and use of
the functions by name:

lookup :: String -> (Int -> [Int])

lookup "a" = MyModule.a

lookup "b" = MyModule.b

...

Is there a more idiomatic Haskell way of going about this?  I can see
there's a fundamental tension between a runtime choice of function
name expressed as a string and the need for compile-time known
function choice.

Thanks,

Stu