StrictDataFail.hs

{-# language BangPatterns #-}

module Main where

import Debug.Trace (trace)

data Source = Source Char Bool
data Intermediate = Intermediate !Char !Bool
data Destination = Destination Char Bool

process1 :: Source -> Intermediate
process1 (Source char bool) = Intermediate char bool -- or do something more complicated

process2 :: Intermediate -> Destination
process2 (Intermediate char bool) = Destination char bool -- or do something more complicated

-- put some data into a lazy type so we will not do any unnecessary expensive computation
source :: Source
source = Source 'c' (trace "expensive computation" False)

destination :: Destination
destination = process2 (process1 source)

result :: Char
result = case destination of Destination char _bool -> char
-- destination is lazy, so let's only get and pay for what we need, right?

main :: IO ()
main = print result

-- output:

-- expensive computation
-- 'c'

Making data strict comes with the same risks as making functions strict by adding seq, $!, bang parameters,..., namely unnecessary computation.