How does Langage.Haskell.TH.report work?

StackOverflow https://stackoverflow.com//questions/9600273

  •  08-12-2019
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Question

Unfortunately, many Template Haskell functions have absolutely no documentation at all. One such function is report. It takes a Bool and a String, and produces a compilation error with the specified string as the error message. Does anybody have any clue what the hell the Bool is for? As best as I can tell, either value does exactly the same thing...

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Solution

If the Bool is True, an error is reported; if it is False, a "warning" is reported, meaning that the template code will continue to run to collect more "warnings."

OTHER TIPS

Looking at the source code, report calls qReport, which is a method of some class called Quasi. This method actually has some damned documentation - though only a tiny snippet. I quote:

Report an error (True) or warning (False) ...but carry on; use fail to stop

So it seems to make my TH splice crash with an appropriate error message, I just need to call fail instead. Hopefully this information will be useful to anyone else trying to figure that out...

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