What does this function definition mean in Haskell?

fn x [] = [] means that if the second argument to fn is an empty list, return an empty list.

fn x (True:ys) = x : fn x ys means that if the second argument to fn starts with True, return a list that starts with the first argument to fn and continues with fn x applied to the remainder of the input list.

fn x _ = [] means that if the second argument to fn is anything not covered by the previous cases, return an empty list. (This line means that the first line in the definition of fn is actually unnecessary.)

To synthesize, fn x takes a list and returns a list made of as many copies of x as there are consecutive True values at the start of the input list.

posted almost 2 years ago

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