[Haskell-cafe] conditional code, was: Template haskell instance checking

Christian Maeder maeder at tzi.de
Thu Mar 2 05:26:45 EST 2006

heatsink wrote:
> Is there a way, in template haskell, to find out what instance 
> declarations exist?  I'm not trying to accomplish anything specific 
> right now, but I can see two potential uses for this.  One is to emit 
> compile-time error messages from TH code which would be more informative 
> than the typechecker's output.  The other is to produce TH functions 
> that conditionally generate code depending on the instances satisfied by 
> their arguments.

Apologies for becoming off-topic now, I've got nothing against template 
haskell (TH), on the contrary, it is a great device to increase genericity.

But I want to warn about conditional code, because this may become a 
nightmare to maintain. I.e. we (at Bremen) have (few) bits in TH that 
check if we run under windows or unix (but no one knows if the windows 
part is still working). Furthermore we support a few older (still 
recent) ghc versions with conditional code (like ghc itself can be 
bootstrapped with different older version).

Plugging in various libraries using Cabal is a nice feature, but imagine 
if you would start to program depending on the currently installed 
packages (and visible instances)! (Fortunately, this would be hard to do 

So (despite cabal and the desire for a minimal kernel) I hope that ghc 
will continue to supply a large number of more or less "standard" 
packages or modules that can safely be expected to exist in (almost) 
every installation. (Therefore I've suggested to keep Data.Set and 
Data.Map in the base package, but another "standard" package may be 
fine, too). Otherwise truly application specific 3rd party cabal 
packages may all come with their own "Prelude extensions" rather than 
reusing increasingly solidified Haskell basics (that are hopefully not 
disputed too much by the design of Haskell-Prime).

If you agree with me, great, ignore this posting.

Respectfully Christian

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