Joachim Breitner's Homepage

More than five years ago I blogged about the “configuration problem” and a proposed solution for Haskell, which turned into some Template Haskell hacks in the seal-module package.

With the new GHC proposal process in plase, I am suddenly much more inclined to write up my weird wishes for the Haskell language in proposal form, to make them more coherent, get feedback, and maybe (maybe) actually get them implemented. But even if the proposal is rejected it is still a nice forum to discuss these ideas.

So I turned my Template Haskell hack into a proposed new syntactic feature. The idea is shamelessly stolen from Isabelle, including some of the keywords, and would allow you to write

context fixes progName in
  foo :: Maybe Int -> Either String Int
  foo Nothing  = Left $ progName ++ ": no number given"
  foo (Just i) = bar i

  bar :: Int -> Either String Int
  bar 0 = Left $ progName ++ ": zero no good"
  bar n = Right $ n + 1

instead of

foo :: String -> Maybe Int -> Either String Int
foo progName Nothing  = Left $ progName ++ ": no number given"
foo progName (Just i) = bar progName  i

bar :: String -> Int -> Either String Int
bar progName 0 = Left $ progName ++ ": zero no good"
bar progName n = Right $ n + 1

when you want to have an “almost constant” parameter.

I am happy to get feedback at the GitHub pull request.