Try this instance instead. instance (a ~ [Turtle.FilePath]) => PrintAllType (IO a) where When you try to use `ls` at some type `IO a`, instance resolution won't instantiate `a`, and thus it will not match `IO [FilePath]`. However, here, it will match `IO a`, and after having picked that instance, it gets to unify `a ~ [FilePath]`. Li-yao On 07/16/2017 03:52 AM, Cody Goodman wrote:
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 https://rosettacode.org/wiki/Variadic_function#Haskell 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
Thanks,
Cody
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