`cmd` does not work with `String` argument

[sgf-dma]

Hi.

cmd does not work with String argument, because ghc uses
ShellCmd ([arg] -> result) instance and looks for CmdArg Char instance.

  {-# LANGUAGE OverloadedStrings #-}
  {-# LANGUAGE ExtendedDefaultRules #-}
  {-# OPTIONS_GHC -fno-warn-type-defaults #-}
  import Shelly
  import qualified Data.Text as T
  default (T.Text)

  -- Does not work.
  strCmd :: Sh ()
  strCmd  = cmd "ls" ("." :: String)

  -- Works as expected.
  strCmdL :: Sh ()
  strCmdL = cmd "ls" ["." :: String]

fails with error

[4 of 4] Compiling Main             ( 1.hs, interpreted )

1.hs:9:11: error:
    • No instance for (CmdArg Char) arising from a use of ‘cmd’
    • In the expression: cmd "ls" ("." :: String)
      In an equation for ‘strCmd’: strCmd = cmd "ls" ("." :: String)

This may be fixed, if we make

instance CmdArg a => CmdArg [a] where

instead of

instance (CmdArg arg, ShellCmd result) => ShellCmd ([arg] -> result) where

This requires changing the type of toTextArg too, though.

[sgf-dma]

The fix i'm talking about is #144 .

[andreasabel]

This bug bit me when upgrading shelly to 1.9. Invocations of cmd that worked before suddenly failed.
The CHANGELOG does not mention such incompatibilities, but should!?

[cunningdefenestrator]

I ran into this as well. It's quite annoying because the docs specifically call out FilePath arguments as a reason to use cmd over run.

The original branch seems to have gone by the wayside, but I have an updated branch (the solution is largely the same as @sgf-dma 's, with some tests), if the maintainers are still open to fixing this.