[Haskell-cafe] Questions about haskell CPP macros

Stephan Friedrichs deduktionstheorem at web.de
Mon Jul 13 13:27:31 EDT 2009

Matthew Elder wrote:
> {-# LANGUAGE CPP #-}
> main = putStrLn (__FILE__ ++ ":" ++ show __LINE__)
> This outputs:
> test.hs:2
> Unfortunately, if your file is in a hierarchy of folders, this flat file
> name doesn't give much context. Is there a macro to find out the current
> module? IE if I had a module Foo.Bar.Car.MyModule, I would like to be
> able to output something like this on error:
> Foo.Bar.Car.MyModule:2

As mentioned by Claus, template-haskell offers a solution. But in some
cases, this is an overkill; consider using Control.Exception.assert, it
will provide module and line information without having to use CPP:

myHead :: [a] -> a
myHead (x:_) = x
myHead []    = assert False undefined

> [...]



Früher hieß es ja: Ich denke, also bin ich.
Heute weiß man: Es geht auch so.

 - Dieter Nuhr

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