[Haskell-cafe] How do I do conditional tail recursion in a monad?

[Jules Bean]

DavidA wrote:
> So I figure untilM should look something like:
> untilM :: Monad m => (a -> Bool) -> (a -> m a) -> a -> m a
> untilM p f x = return (if p x then x else untilM p f (f x))
> The problem is that the two branches of the conditional have different types. 
> If I try to remedy that by changing "then x" to "then return x", it still isn't 
> happy.
>
>   

You are applying return to the result of untilM.

This makes a m (m a). The following is closer:

*Main> :t let untilM p f x = if p x then return x else untilM p f (f x) 
in untilM
let untilM p f x = if p x then return x else untilM p f (f x) in untilM
:: (Monad m) => (a -> Bool) -> (a -> a) -> a -> m a


...but here 'f' is a pure function, not a monadic action. If you want f 
to be a monadic action then you want:

*Main> :t let untilM p f x = if p x then return x else untilM p f =<< f 
x in untilM
let untilM p f x = if p x then return x else untilM p f =<< f x in 
untilM :: (Monad m) => (a -> Bool) -> (a -> m a) -> a -> m a


(if you prefer do notation to explicit binds, rewrite "untilM p f =<< f 
x" as "y <- f x ; untilM p f y")


> (My real question is, how do I do conditional tail recursion in a monad?)
>   

actually I don't think that was your real question :) Your real question 
was about conditional recursion in monads, but had nothing to do with 
tailness or otherwise. Tail recursion is a concept which has 
implementation relevance in an eager language, but is often a 
red-herring in a lazy language.

Jules