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What does this function definition mean in Haskell?
What does this function definition mean in Haskell?
fn x [] = []
fn x (True:ys) = x : fn x ys
fn x _ = []
1 answer
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.
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