Guard

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Haskell

List comprehensions can use guards to restrict or filter the values produced by earlier generators.

  • [x | x <- [1..10], even x]
  • The list [2,4,6,8,10] of all numbers x such that x is an element of the list [1..10] and x is even.

If you use a linq list comprehension or comprehension syntax that will be called WHERE.

Using a guard we can define a function that maps a positive integer to its list of factors:

  • factors :: Int -> [Int]
  • factors n =
  • [x | x <- [1..n], n mod x ==0]

  • factors 15

  • [1,3,5,15]
  • we can define a function that decides if a number is prime:
  • prime :: Int -> Bool
  • prime n = factors n == [1,n]

  • prime 15

  • False
  • define a function that returns the list of all primes up to a given limit:
  • primes :: Int -> [Int]
  • primes n = [x | x <- [2..n], prime x]

(FP101x)

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