Question

I will try to state this question as short and to the point as possible.

I expected this computation to complete after inputting three characters to the command line. However, it does not. I am curious as to why this is.

return . take 3 =<< sequence (repeat getChar)

Edit:

I should add that I am trying to separate selection from generation and therefore wish to produce an infinite stream of characters that I select from.

Answer 1

1. Effects to the right of =<< complete before effects to the left of =<< begin.
   • This means that if the action to the right of =<< would run forever then the entire action will run forever.
2. sequence takes a list of actions and gives back an action that performs all those actions.
   • If you give sequence an infinite list of actions, it will run forever.
3. repeat takes a value, and gives an infinite list of that value, repeated.

Do you see now why you get the behaviour you experience?

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This would work, because there is no infinite list:

sequence (take 3 $ repeat getChar)

But presumably your real selection function is more complicated than “take the first three”. What is it?