[Haskell-cafe] Generating Code

[Geoffrey Mainland]

On 12/10/2011 09:38, Iustin Pop wrote:
> On Fri, Dec 09, 2011 at 10:30:18PM +0100, L Corbijn wrote:
>> The major set of problems for using template haskell is that it
>> doesn't have the correct features, or better said it tries to solve
>> another problem. Template haskell generates code into an existing
>> module, while for this problem there is no module yet to generate it
>> into. Of course I could generate those modules and let template
>> haskell make the FFI imports, but then the problem remains how to
>> generate those modules. So template haskell seems (as I see it) to
>> solve the problem of writing almost the same code twice by generating
>> it from some parameters coded in some source file. Another problem is
>> that the export and import lists of the modules need to be generated
>> too and this seems not an option for TH.
> 
> On Fri, Dec 09, 2011 at 04:27:31PM -0600, Antoine Latter wrote:
>> For my case, template haskell can't create modules, and template
>> haskell solves a different problem - I've not interested in creating
>> Haskell declarations from Haskell declarations - I'm interested in
>> creating Haskell modules from an external, formal,  specification. In
>> a sense I'm compiling to Haskell.
> 
> This answer is for both the above quotes. While TH is not perfect (and
> sometimes tedious/difficult to write in), it's not restricted to simply
> generate code based on some parameters in an existing Haskell file.
> 
> It cannot generate modules, true, but other than that you could have a
> module simply like this:
> 
>   module Foo where
> 
>   import …
> 
>   $(myBuilder)
> 
> Where myBuilder doesn't take any parameters, just reads some external
> (XML, text file, whatever) and build the code from scratch.
> 
> I might misunderstand the problem, but I think that you _could_ use TH
> for "compiling" to Haskell, as long as you have a Haskell parser for the
> external/formal spec.
> 
> regards,
> iustin

There are the haskell-src-exts-qq and haskell-src-meta packages on
hackage that will get you partway to generating source code for a
Haskell module. Full support for Haskell quasiquotation would require a
modified version of haskell-src-exts (to properly handle antiquotation).

The problem with TH is that it runs when the module is compiled. Reading
a spec in from a file within a TH quote is possible, but it would be
much nicer to be able to write a code generator, which a full Haskell
quasiquoter would allow.

Maybe someone is interested in a side-project? :)

Geoff