Tutorials : Chapter 3

In the first example of Selective Receive this line won't typecheck:

mapM_ (say . show) [first, second, third]

because the types of first, second and third are not the same.

first :: String
second :: Maybe String -- (corrected in PR#35)
third :: ProcessId

A possible simple solution would be to manually use

(say.show) first
(say.show) second
(say.show) third

Or otherwise construct a heterogenous lists(using GADTs or existential quantification) and then use mapM_ which seems cumbersome and unnecessary.