A generator is the list-comprehension clause that supplies values for the expression on the left. The expression is evaluated for every generated value that satisfies the guards. section 5.4.1
Generate and filter values
[x + 1 | x <- [1..10], x <= 5] produces [2,3,4,5,6]. The generator supplies x from [1..10], while the guard keeps only values through 5. section 5.4.1
Generators support several useful list-comprehension patterns:
- Multiple generators combine their values using a Cartesian product, so each left-hand expression runs for every combination. section 5.4.1
- Generators can construct practical candidate lists, such as adding, removing, or modifying letters in a word. section 5.4.1
- A pattern in a generator can filter values: a failing match skips the value, as in [x | Just x <- xs]. section 5.4.1
ghci> [ x + 1 | x <- [1..10] ] :: [Int] [2,3,4,5,6,7,8,9,10,11] ghci> [ x + 1 | x <- [1..10], x <= 5 ] :: [Int] [2,3,4,5,6]
Interactive examples show a generator supplying values and a guard restricting which generated values reach the result. section 5.4.1
