Should exhaustiveness testing be on by default?

Peter Hercek phercek at
Thu May 21 09:15:23 EDT 2009

Ok, I went with the preprocessor solution only. It is simple, stupid and 
works well enough ... and template haskell alternative needs it anyway 
not to be too unportable.

Both template haskell alternatives reported "Pattern match(es) are 
non-exhaustive" of their own. The second alternative moreover needs a 
change of '$ case True of False -> "srcloc"' to '$ case True of False -> 
undefined' to be usable.

The warning problem is critical by its own since the goal of using it is 
to selectively disable the very same warning for a specific case 
statement. Although the warning can be eliminated probably in the first 
template haskell alternative. Not sure since I do not know template 
haskell. As well as I still do not know how to write a haskell function 
in C-- which is the reason there is no :next command in ghci yet :)


Claus Reinke wrote:
>> The second solution requires QuasiQuotes, so I do not know. If I 
>> would want to compile with a different compiler it would break. If 
>> srcloc can be defined as a simple token (not requiring special 
>> extensions at places where it is used) then I could define it to an 
>> empty string in some low level module if trying to compile with a 
>> different haskell compiler which does not know srcloc.
> You can do better than that, if you combine the QuasiQuotes hack with
> the CPP hack (I've also simplified the srcloc handling by adding a 
> version
> of error that adds source location info, moving the exception 
> manipulation
> out into SrcLocQQ, avoiding the need for Debug.Trace alltogether).
> The portable version does get a bit uglier because you need macros, 
> not functions (you'll probably want to check for GHC version or 
> -better, but not supported- QuasiQuotes availability). Also, CPP only 
> gives you the line number, not the position, but that is better than 
> nothing, and often sufficient.
> Still, it would be much nicer if GHC inserted the location info at the
> call sites if a pragma at the definition site asked it to do so. Then 
> this
>    {-# SRCLOC f #-}
>    f Nothing = "okay"
>    f _ = error "f applied to not-Nothing in: "
> could be equivalent to the code below, without QuasiQuotes or CPP
> or ERRORSRC all over the place. But such niceties are on hold while 
> the discussion of even nicer help is ongoing.. (which is partly justified
> because we cannot easily build nicer abstractions over a barebones
> solution, due to the macro vs function issue, so the design does need
> thought). Perhaps the code below is sufficient as an interim workaround.
> Claus
> -----------------------------
> {-# LANGUAGE CPP #-}
> {-# LANGUAGE QuasiQuotes #-}
> #ifdef __GLASGOW_HASKELL__
> #define SRCLOC [$srcloc||]
> #define ERRORSRC [$errorSrc||]
> #else
> #define SRCLOC (show (__FILE__,__LINE__))
> #define ERRORSRC (\msg->error $ msg++SRCLOC)
> #endif
> import SrcLocQQ
> f errorSrc Nothing = "okay"
> f errorSrc _       = errorSrc "f applied to not-Nothing in: "
> main = do
>  print $ f ERRORSRC Nothing
>  print $ f ERRORSRC (Just ())
>  print $ SRCLOC
> -----------------------------
> {-# LANGUAGE TemplateHaskell #-}
> module SrcLocQQ where
> import Language.Haskell.TH.Quote
> import Language.Haskell.TH
> import Control.Exception
> srcloc = QuasiQuoter  (\_->[| mapException (\(PatternMatchFail fail)->
>                          let srcloc = reverse (dropWhile (/=':') 
> (reverse fail))
>                          in PatternMatchFail srcloc)
>        $ case True of False -> "srcloc" |])
>  (error "pattern srclocs not supported")
>            errorSrc = QuasiQuoter
>  (\_->[| \msg->mapException (\(PatternMatchFail fail)->
>                                let srcloc = reverse (dropWhile (/=':') 
> (reverse fail))
>                                in PatternMatchFail (msg++srcloc)) 
>              $ case True of False -> "srcloc" |])
>  (error "pattern srclocs not supported")
> -----------------------------

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