2008/2/8 Jeff Polakow
readM with fail defaults to ioError for almost all Monads, and so admits many dangerous programs, which is against the intent of the proposal in the first place.
ioError is only in the IO monad and is catchable. I think the problem is when fail defaults to error which can be anywhere and is not catchable.
I think Don meant "error". The default definition of fail in the Monad class is "error".
It seems with Conor's suggestion of a
maybeReturn :: MonadPlus m => Maybe a -> m a maybeReturn = maybe mzero return
we can still have the by-default-safe maybeRead, that doesn't admit exception throwing opportunities.
Isn't the MonadPlus approach also by-default-safe?
Safe, yes, but is it more useful? (I'm tempted to argue that the way
MTL conflates mzero/mplus with throwError/catchError is unfortunate,
but that's another discussion.)
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Dave Menendez