Get Programming with Haskell

Lesson 32. The list monad and list comprehensions

After reading lesson 32, you’ll be able to

• Use do-notation to generate lists
• Filter results in do-notation by using guard
• Further simplify do-notation with list comprehensions

At the end of the preceding lesson, you saw that List is an instance of Monad. You saw only a simple example of using List as a Monad to process a list of candidates.

Listing 32.1. The assessCandidateList function from the previous lesson

assessCandidateList :: [Candidate] -> [String]
assessCandidateList candidates = do
   candidate <- candidates                        1
   let passed = viable candidate                  2
   let statement = if passed                      3
                   then "passed"
                   else "failed"
   return statement                               4

• 1 By using <- , you’re able to treat your list of candidates like ...