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

On Feb 8, 2008 4:56 AM, John Meacham wrote:

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

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

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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.

That's why I use "fromMaybe" (or "maybe"). Why narrow it down to a call to some function when you can narrow it down to a specific call to a function? It's a pity there isn't something like __LINE__ in Haskell itself.

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.

We already have a function that produces an opaque error string to pass to the user: read. -- Dave Menendez http://www.eyrie.org/~zednenem/