CODEWITHSUNDEEP
haskellfunctional-programming
AMINA ARSHAD
AMINA ARSHAD

Reputation: 41

Produce an infinite list of identical elements in Haskell

In Haskell programming language, how can we produce an infinite list of identical elements.What is the code in Haskell in which user inputs some number and infinite list of that number generated. I had practiced but it was not successful. My logic is below: Code:

repeat :: a -> [a]
repeat x = xs where 
          xs=x*xs

Upvotes: 2

Views: 2117

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 476574

You are quite close, but the strange thing here is that you write x*xs, (*) is an operator that is usually used to multiply numbers, it is a function that is a method of the Num typeclass.

In case you construct a list, you use the list data constructors. The two data constructors are [] :: [a] (the empty list), and (:) :: a -> [a] -> [a]. Here it makes sense to use the (:) operator: we prepend xs with x, so we can write:

repeat :: a -> [a]
repeat x = xs
    where xs = x : xs

Here we thus construct for a given x parameter, a linked list that looks like:

+-------+
|  (:)  |<-.
+---+---+  |
| o | o |  |
+-|-+-|-+  |
  v   `----'
  x

So a "cons" element where the tail refers to itself. As a result if you perform some list processing, it is possible that the memory usage remains constant (without garbage collection) and Haskell thus keeps looping in the same cons object.

Upvotes: 9

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