[Haskell-cafe] Parsec Preprocessor

[Alessio Valentini]

Dear Haskellers

This is gonna be my first post on here : ) .

I am developing some tools to interact with Chemistry packages, and I
basically have to write input files for those programs.

I would like to be able to write those input in a better way inside the hs
code. For example this would be perfect ( {{{+ and +}}} are just labels to
mark the "special syntax"):

--------------a.hs--------------------------------
main = do
            writeFile "inputOfChemPack" string

string = {{{+
This is the actual input file
Formatted like this
with some %variable
+}}}

variable = "foo"
--------------------------------------------------

I have written a parser that is reading the code and outputs a file like
this (it can handle special characters and variables that are not strings,
but let's keep it simple here):

----------------b.hs------------------------------
main = do
            writeFile "inputOfChemPack" string

string = "This is the actual input file\nFormatted like this\nwith some "
++ variable ++ " "

variable = "foo"
--------------------------------------------------

Now... this file can be compiled and it does exactly what I want: it prints
a file like this:

--------inputOfChemPack-----
This is the actual input file
Formatted like this
with some foo
--------------------------------------

Now the big question... right now I am doing this with some bash scripts
(run the parser on the file and then compile its output)... but... is it
possible to automatize this using my parser as a sort of preprocessor?

the "dream" would be to make an extension that just applies the parser
automatically before compilation, such as:

-----------------a.hs-----------------------------
{-# extensionName #-}

main = do
            writeFile "inputOfChemPack" string

string = {{{+
This is the actual input file
Formatted like this
with some %variable
+}}}

variable = "foo"
--------------------------------------------------

$ ghci a.hs
GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( a.hs, interpreted )
Ok, modules loaded: Main.
*Main> string
"This is the actual input file\nFormatted like this\nwith some foo"

I have been browsing stuffs about quasiquotation and preprocessors, but I
have the feeling that is a quite simple task compared to those solutions.

Any ideas? How can I solve this problem?

Thank you

Alessio
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