Discussion: adding displayException to Exception class

[Simon Peyton Jones]

As I recently commented on this list[1], the Show typeclass is overloaded with multiple meanings (serialization, debug info, and user-friendly data display). The general consensus seems to be that the official semantics for Show should be for serialization (as paired up with Read).

Really?  My instinct is otherwise: to use Show for human-readable display, and Binary for serialisation.  Show/Read is a terribly inefficient serialisation format; as soon as you want to do it for real you end up with Binary anyway.  And Show is already well established for human-readable purposes – that was one of its primary original purposes.

Simon

From: Libraries [mailto:libraries-bounces at haskell.org] On Behalf Of Michael Snoyman
Sent: 28 October 2014 23:44
To: libraries; Christopher Done
Subject: Discussion: adding displayException to Exception class

I don't want to make a format proposal yet, just open up discussion on an issue, and see how others feel about it.
As I recently commented on this list[1], the Show typeclass is overloaded with multiple meanings (serialization, debug info, and user-friendly data display). The general consensus seems to be that the official semantics for Show should be for serialization (as paired up with Read).

However, there's at least one use case in base which explicitly uses Show for non-serialization purposes: the Exception class. All instances of Exception are required to have an instance of Show, but there's no way to actually serialize exceptions generally[2]. So by construction and by practice, the Show instance for Exception is used exclusively for the latter two categories I mentioned (debug info and user-friendly data display).

As a result of that, it's quite common[3] to define a Show instance for exception types that is *not* serializable, but rather user friendly. This however seems to contradict what is accepted as good practice in general.

I have a possible solution that I'd like to propose as a strawman: add a new method to the Exception typeclass:
    displayException :: e -> String
    displayException = show
The name is up for debate, but I chose `display` since it seems to already be used elsewhere for user-friendly output. Other possible choices are showException or prettyException. The default implementation will reuse the Show instance, so no user code will be broken by this[4].

In the short term, the only other change I'd recommend is that the default exception handler use `displayException` instead of `show` for printing uncaught exceptions. Longer term, after deprecation cycles, we could consider removing the Show superclass from Exception.
Again, this isn't a real proposal (yet), I'm more interested now in what people think about such a change.

Michael

[1] http://www.haskell.org/pipermail/libraries/2014-October/023957.html
[2] Both due to how extensible exceptions work, and due to the lack of a Read superclass.
[3] This discussion started when Chris Done and I were working with the tar package, which defines custom renderings for FormatError. I've implemented this same technique in some of my libraries, and I'm sure there are many other examples on Hackage.
[4] Barring the standard issue of new identifiers clashing with identifiers defined locally.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/libraries/attachments/20141029/14ab4a14/attachment-0001.html>