[Haskell-cafe] better error message for "undefined"

Andrew U. Frank frank at geoinfo.tuwien.ac.at
Thu Sep 10 08:02:27 EDT 2009

the haskell prelude introduces the type undefined and contains a remark to the 
effect that compilers are expected to intrudocue a meaningful error message. 
GHC just prints 'Prelude undefined' which is not very informativ.

is there a way to have different kinds of undefined? such that i get a message 
which one was hit. something like writing in the code 'unefined 'my error 123' 
which prints 'undefined - my error 123' when it is accidentally hit.

i searched the web, but found nothing. any suggestions? 
thanks a lot!


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