I am a huge newbie to Haskell, I actually just started 10 minutes ago. I am trying to figure out how to define a variable inside a function. Lets say I have the function
foo :: Int -> Int
foo a =
b = a * 2
b
-- Yes, I know, it doesn't do anything interesting
When I run it in GHCi I get a syntax error! How can you define a variable inside a function?
There are two ways to do this:
foo a = b where b = a * 2
foo a = let b = a * 2 in b
In most cases, the choice between them is an aesthetic rather than technical one. More precisely, where may only be attached to definitions, whereas let ... in ... may be used anywhere an expression is allowed. Both where and let introduce blocks, making multiple internal variables convenient in both cases.
where b = a * 2, c = a * 3 or would you use an entirely different "where" statement?
where/let will do. Use ; to separate declarations on the same line, or read about the indentation rules to use multiple lines. (This is the meaning of the last sentence of my post.)
foo a = b where b = a * 2 basically translates to foo a=(/b->b)a * 2 ?foo :: (Int, Double); foo = (x, x) where x = 3 works fine, but foo :: (Int, Double); foo = (\x -> (x, x)) 3 does not.Disregarding technical correctness, the answer is "sort of".
I think it's better to think of a variable as a function of zero arguments evaluating to a given value.
module Main where
import System.IO
foo :: Integer -> Integer
foo a =
b where
b = a * 2
main = do
putStrLn $ show $ foo 10
