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haskelluniplate

Count `Pat`s in a Module

I need to count the number of Pat in a haskell Module. I know the simplest way is to pattern match on each level of the AST, which will result in a huge function that looks like the entire AST. I believe there's some way to take advantage of typeclasses like Functor or the State Monad to lean on some existing function that walks the tree (like prettyPrint) and trace a counter along, but I'm not sure how it works exactly.

Solution

  • It's very easy using uniplate:

    import Data.Data
import Data.Generics.Uniplate.Data
import Control.Monad
import Language.Haskell.Exts

findPats :: Data a => a -> [Pat]
findPats = universeBi

test = do
  content <- readFile "Simple.hs"
  case parseModule content of
    ParseFailed _ e -> error e
    ParseOk a       -> do
      forM_ (findPats a) $ \p -> do
        putStrLn $ "got a pat: " ++ show p

    Essentially it's just the universeBi function.