Re: Proposal: Add Text.Read.maybeRead :: Read a => String -> Maybe a

On Fri, Feb 08, 2008 at 12:17:31AM -0500, David Menendez wrote:

On Feb 7, 2008 6:12 PM, Ross Paterson wrote:

...

I think this is a good idea, though I normally write the equivalent of

maybeRead :: Read a => String -> Maybe a maybeRead s = case reads s of [(x, rest)] | all isSpace rest -> Just x _ -> Nothing

and would prefer not to generalize it to any monad. fail is a wart, not a design pattern.

I also prefer Maybe to fail. Error strings are only useful if you're ignoring them or passing them to the user without interpretation.

say that next time you get a mysterious "fromJust: Nothing" error with no context, error messages in haskell are quite an issue as is, _any_ useful information is good, passing them on to the user without interpretation is loads better than not having any clue what went wrong. It is what you want to do because the fact that it was a 'readM' that failed is extremely useful for figuring out what went wrong. It is a straightforward generalization that is very useful pragmatically. John -- John Meacham - ⑆repetae.net⑆john⑈