Syntax example for haskell-based preprocessor idea.

Prehaskell

{{ 
  --Arbitrary haskell code in double curly brackets
  --Top-level definitions are available throughout text
  emph s = "*" ++ s ++ "*"
}}

This will be rendered raw.
{{
  -- String-typed expressions will be printed into the final text wherever they appear
  emph "This will be emphasized"
  -- IO String-typed expressions will be evaluated, in the order that they appear in the file, and the values printed
  emph `fmap` getContents
}}

Another thing that should be possible(paraphrasing a bit).

{{
  viewDiagram :: Diagram -> String -> IO String
  -- Yes, Diagram is now a type, shut up.
  viewDiagram d s = do
    saveDiagram d s
    return $ "\insertDiagram{"++s++"}"
}}