Catching errors thrown with `error`?

Question

There are some stdlib functions that throw errors on invalid input. For example:

Prelude> read "1o2" :: Int
*** Exception: Prelude.read: no parse

I would like to wrap it to return a Either e a instead. How can I do that?

score 14 · Accepted Answer · answered Jun 07 '12 at 07:01

14

There is no spoon. You didn't hear it from me.

For this particular example, though, you should use reads instead.

answered Jun 07 '12 at 07:01

Daniel Wagner

I like the way you phrased it. :) Thank you for the answer! – missingfaktor Jun 07 '12 at 07:06

score 2 · Answer 2 · answered Jun 07 '12 at 12:12

2

I prefer to turn errors into values:

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

answered Jun 07 '12 at 12:12

Don Stewart

I too prefer that, but the question is about doing it _generally_. `read` is used only as an example. – missingfaktor Jun 07 '12 at 12:15
I think it is generally true that, for any function which throws an error, there will be a "safe" version of it. – Tyler Jun 07 '12 at 16:05

2 Answers2