Readline read_history and write_history addition

Ian Lynagh igloo at
Tue Jan 22 19:51:11 EST 2008

On Wed, Jan 23, 2008 at 01:49:34AM +0200, Yitzchak Gale wrote:
> Ian Lynagh wrote:
> > People using MonadIO can convert this into the variant that doesn't
> > throw an exception by replacing
> >     readlineFunction args
> > with something like
> >     (try $ readlineFunction args) >> return ()
> That's in the IO monad, so not always available.

Everywhere you can write
    readlineFunction args
you can write
    (try $ readlineFunction args) >> return ()

The only way you can have problems is if there is a library (that you
don't control), which exports a (MonadIO m => m a) that internally calls
    readlineFunction args
and doesn't catch exceptions. Personally I'd say that that is a bug
in that other library, and it ought to be catching the exception and
either ignoring it, returning some sort of sum type, or also
constraining m to be in some sort of MonadError monad.

> If we start throwing IO exceptions for common and minor
> occurrences like no readline history available

Pretty much any actual IO you do has this problem, e.g. readFile on
a non-existent file.


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