[Haskell-cafe] Mysterious monads

[Henning Thielemann]

On Sun, 27 May 2007, Andrew Coppin wrote:

> such that a Reader is created with an initial list, and the read
> function fetches 1 element out of that list. That is, the expression "x
> <- read" will take the head element of the list and put it into x,
> keeping the tail to be read later.
>
> (Oh yeah - and apparently that clashes with Prelude.read. Oh well!)
>
> I can't figure out how to implement this... The closest I managed was to
> make a Reader object also contain a function that tells (>>=) what to do
> to the Reader object you're binding against... But that seems to be
> horribly buggy.

Confusingly MonadReader and MonadWriter in the Monad Template Library are
not quite counterparts. Reader does not consume its input, Writer does not
overwrite its output.