dataTypeConstrs gives unhelpful error message

Take the following program:

import Data.Data
foo = dataTypeConstrs (dataTypeOf (0 :: Int))
-- raises: *** Exception: Data.Data.dataTypeConstrs

The implementation of dataTypeConstrs has the type available as a string at this point, so could easily say:

Exception: Data.Data.dataTypeConstrs is not supported for Int, 
           as it is not an algebraic data type

changed weight to 5

added Tbug Trac import core libraries labels

changed the description

Good idea! Could you send a patch? Thanks!

Simon

mentioned in issue #5454 (closed)

changed milestone to %7.6.1

changed milestone to %7.6.2

assigned to @trac-klangner

Attached file messages.patch.gz (download).

Patch

Attached file 0001-Better-error-message-for-function-with-DataType-para.patch (download).

unassigned @trac-klangner

Dear Krzysztof,

thanks for the patch. Note that SPJ made a more elaborate proposal in the ticket, namely printing the name of the type with the error message. Do you think you can update your ticket to print a message similar to what Simon proposed?

assigned to @trac-klangner

I can try :-)

I can create the following message: Data.Data.dataTypeConstrs is not supported for Prelude.Int, as it is not an algebraic data type

As you can see there is **Prelude.Int** not **Int**. This name is returned by **dataTypeName**.

I don't know how to get the name without Prelude prefix.

Attached file 0001-Better-error-message.-Ticket-5412.patch (download).

assigned to @nomeata and unassigned @trac-klangner

I just checked the API as well, and there does not seem to exist a clean way of getting the unqualified name. But as this is just an error message, I think it’s fine.

I’ll validate and push.

assigned to @trac-klangner and unassigned @nomeata

Wait: The second patch only modifies dataTypeConstrs while the first patch changes the error message of several functions. Oversight?

Also, I’d add a “.” at the end of the message, after all, it is a full sentence.

Yes you are right. The problem is that this kind of message can't be used in other places. So I was not sure if I should change other functions as well. On the other hand it would be nice to improve some other error messages since probably sooner or later this will return as new ticket.

How do you think would be better?

1. Change only function from this ticket description
2. Change only messages where ADT is expected (4 places) and leave other messages intact.
3. Try to improve other error messages in this module as well.

Third option will require more work and I'll probably need some help with the message texts.