Ketil Malde wrote:

"Russ Lewis" writes:

...

Another newbie question: Is it possible to catch _|_ - that is, to encounter it, handle it and continue?

Since _|_ is the value of a non-terminating computation, this requires you to solve the halting problem. GHC does occasionally detect loops, but for the general case, don't hold your breath :-)

-kzm

If all you are interested in is catching calls to error, they can be caught like normal exceptions. Here is some code from Network.XmlRpc that catches error calls and handles them in an error monad: type Err m a = ErrorT String m a -- | Evaluate the argument and catch error call exceptions errorToErr :: Monad m => a -> Err m a errorToErr x = let e = unsafePerformIO (tryJust errorCalls (evaluate x)) in ErrorT (return e) Pretty nasty, using evaluate and unsafePerformIO in the same function. The essential part is (tryJust errorCalls). /Bjorn Bringert

"Russ Lewis" writes:

Oh. I was figuring that the runtime would detect _|_ whenever evaluation requires that it calculate a given expressoin, and that expression is currently being evaluated...that is, some subset of an expression evaluates to the expression itself.

You mean like in fibs = 1:1:zipWith (+) fibs (tail fibs) ? :-) Actually, (and more seriously) I suspect this is the kind of loop detection that makes GHC go "Exception: loop" (which you may be able to catch; I don't know the details since I just let my programs crash instead :-) Prelude> let x = x Prelude> x *** Exception: <<loop>> -kzm -- If I haven't seen further, it is by standing in the footprints of giants