问题
I'm very new to Haskell, and I have a simple question.
What function can I use with a and b that will result in a, b times.
Example:a = 4 | b = 3
Would return:[4, 4, 4]
Thanks!
回答1:
replicate:
replicate 3 4
will be:
[4,4,4]
When you know what's the type of the function you need (in this case it was quite obvious that the function you needed had a type similar to Int -> a -> [a]) you can use Hoogle in order to find it.
回答2:
You could also use recursion (although the solutions above should be preferred of course):
rep a 0 = []
rep a b = a : rep a (b-1)
回答3:
Of course peoro is right, you should use replicate.
However, a very common pattern for such tasks is to construct an infinite list, and take as much as you need from it (either using take or takeWhile):
rep a b = take b $ repeat a
Another (more educational than practical) approach is to use a list of the right length, and map all elements to a:
rep a b = map (const a) [1..b]
An very inefficient, but interesting version is
rep a b = until ((b==).length) (a:) []
回答4:
.. or simply something like that
> take 3 [4,4..]
> [4,4,4]
来源：https://stackoverflow.com/questions/5592624/repeat-in-haskell