[Haskell-beginners] comment on this debugging trick?

David McBride toad3k at gmail.com
Fri Nov 27 19:05:58 UTC 2015

If you compile with -prof you can use the traceStack function from
Debug.Trace.  That's something.

On Fri, Nov 27, 2015 at 8:56 AM, Dennis Raddle <dennis.raddle at gmail.com>

> When I design my code, I am aware of the properties I expect my internal
> data representations to have, or I want input data to conform to my
> expectations. For example, right now I'm writing code that reads MusicXML.
> MusicXML is a crazy language, way too complicated for what it does, and the
> music typesetters that export MusicXML all do their own idiosyncratic
> things with it. I'm only reading the output of one typesetter, Sibelius,
> and although I don't know the internals of Sibelius, I can make some
> assumptions about what it's going to produce by a few examples, plus my
> application doesn't use the full range of MusicXML. I don't need to handle
> every case, is what I'm getting at.
> However, if I should have been wrong about my assumptions when I wrote the
> code, I don't want my program to behave erratically. I want to find out
> exactly what went wrong as soon as possible.
> Previously, I was including a lot of exception throwing, with each
> exception having a unique message describing what had happened, and in
> particular describing where in the code it is located so I can find the
> problem spot
> If I throw generic error messages like "Error: problem parsing" and I have
> many of those located in the code, I need the debugger to find it. Same if
> I use things like "fromJust" or "head"
> I haven't had good experiences using the debugger. Maybe that's my fault,
> but I like that I can locate my unique exceptions and that they are
> descriptive.
> But a month ago I decided this system is pretty ugly. It bloats the code a
> lot, and requires writing a lot of messages for situations that will
> probably never occur.
> I am trying a different method now. I write the code so that surprising
> behavior will result in a case or pattern exhaustion, which produces a run
> time message with a file name and line number. I'm not sure why ghc gives
> the location for a case exhaustion, but not something like "head".
> For example, instead of using head xs I can write
> x = case xs of {y:_ -> y}
> This is a little bloat but not too bad. Most of the time I'm actually
> structuring cases and patterns, and it actually helps me to simplify code
> to think of how to write it with the fewest cases and so that a case
> exhaustion will indicate something pretty specific.
> Any comments welcome.
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