module haskell
imports libstrategolib signatures/- util

signature
  constructors
    TopLevel: Ex -> Ex

rules
  /* Approach: Generate code from the bottom up.
     At every node, we create a pair of the implementation and
     necessary preamble of IO actions.
     We concatenate preambles as we go up.
     Finally, at the toplevel we emit the preamble before returning the
     final value.
   */

  hs: TopLevel((c,p)) -> $[import System.IO
  data IOStream = StdIO

  gets :: Show b => a -> b -> IO a
  gets s d = do
    putStr(show d)
    return s

  main = do
    [p]return [c]]

  hs: Stream() -> ("StdIO", "")
  hs: Int(x) -> (x, "")
  hs: Sum( (c, p), (d, q)) -> ($[([c] + [d])], <conc-strings>(p,q))
  hs: Gets((c, p), (d, q)) -> <hsget>(c,d,<conc-strings>(p,q),<newname>"fosgt")

  hsget: (s, x, p, v) -> (v, <concat-strings>[p, $[[v] <- [s] `gets` [x]],
                                              "\n"])

  hs: To(  (c, p), (d, q)) -> <hsto>(c,d,<conc-strings>(p,q),<newname>"fosto")

  hsto: (x, s, p, v) -> (v, <concat-strings>[p, $[let [v] = [x]], "\n",
                               $[[s] `gets` [v]], "\n"])

  hs: Sequence(l) -> (<last; Fst>l, <map(Snd); concat-strings>l)

strategies

  haskell = bottomup(try(hs))

  // Interface haskell code generation with editor services and file system
  to-haskell: (selected, _, _, path, project-path) -> (filename, result)
    with filename := <guarantee-extension(|"hs")> path
       ; result   := <haskell> selected