User talk:Elias

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Exercises for the enthusiast beginner

A simple exercise, read carefully

This is a definition for the readln function. If you do not know the bind operator (>>=) yet, do not worry about it. p >> q behaves in a way similar to q(p), but it involves side effects. Anyway the error you are asked to find is not related with it.

> readln = h
>    where h = getChar >>= f 
>            where f c = case c of
>                          '\n'      -> return []
>                          otherwise -> h >>= (return . (:) c)


Main> readln >>= print

It works, as expected.

Now hofun is a higher order function obtained by the parameterization of readln, it generalizes its behaviour. The new function readln' calls hofun with the original parameters. It can be checked by replacing the parameters in the body of the function.

> readln' = hofun getChar '\n' (:) []

> hofun r s op e = h 
>    where h = r >>= f 
>            where f c = case c of
>                          s -> return e
>                          otherwise -> h >>= (return . op c)


Main> readln' >>= print

But readln' >>= print does not works printing the input line as readln >>= print does, there is an error, easy to find for the experienced eye. Can you discover it?