Question

Given this line of code (which I first saw in this answer):

pVal :: Num a => a
pVal = sum . map fromIntegral $ ([1..10000000] :: [Int])

If it’s used as multiple types, is the expression completely reevaluated for each type? Is one result for each type kept around?

For example:

pInt :: Int
pInt = pVal

pDub :: Double
pDub = pVal

Answer 1

Technically, the Haskell standard doesn’t specify. In practice, it’s re-evaluated each time. You can verify this yourself using Debug.Trace:

import Debug.Trace (trace)

pVal :: Num a => a
pVal = trace "evaluating..." $ sum . map fromIntegral $ ([1..10000] :: [Int])

Try it in GHCi:

> pVal :: Int
evaluating...
50005000
> pVal :: Int
evaluating...
50005000

However, binding this value to a concrete type will allow it to be re-used.

pInt :: Int
pInt = pVal

Notice there’s no “evaluating…” the second time:

> pInt
evaluating...
50005000
> pInt
50005000