[Haskell-cafe] Wrapping Turtle ls function to work with 0 or 1 arguments (polyvariadic)

Cody Goodman codygman.consulting at gmail.com
Sun Jul 16 07:52:06 UTC 2017

I'm trying to get ls to work in ghci like it does in bash using variadiac
arguments in Haskell. I'm stuck at the moment so I thought I'd send this to
#haskell-cafe for some help. I'm not quite sure how to proceed.

See the main function for what I want the end result to look like. Here is
the code:

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE OverloadedStrings #-}
-- | An example module.
module Example (main) where

import qualified Turtle as Turtle
import Control.Foldl
import Control.Monad.IO.Class

lsDirEx :: MonadIO io => io [Turtle.FilePath]
lsDirEx = do
  Turtle.fold (Turtle.ls "/home") list

class PrintAllType t where -- use PrintAllType from
    process :: [Turtle.FilePath] -> t

-- instance MonadIO io => PrintAllType (io [Turtle.FilePath]) where
instance PrintAllType (IO [Turtle.FilePath]) where
  process [] = do -- ls received no args print current directory
    Turtle.pwd >>= \fp -> Turtle.fold (Turtle.ls fp) list
  process (filePath:[]) = do -- ls recieved one filePath
    liftIO $ Turtle.fold (Turtle.ls filePath) list
  process _ = error "multiple arguments not currently supported"

-- instance (Show a, PrintAllType r) => PrintAllType (a -> r) where
--     process args = \a -> process (args ++ [fmt a])
--       where fmt thing = _ $ Turtle.format Turtle.w thing

ls :: (PrintAllType t) => t
ls = process []

-- | An example function.
main :: IO ()
main = do
  ls -- lists current directory
  ls ("/home" :: String) -- lists /home directory


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