Add partial version of getEnv

[kaoskorobase]

This is a suggestion to add a partial version of getEnv, because I end up defining it in almost every shake build system; maybe others, too?

Something like

getEnv_ :: String -> Action String
getEnv_ name =
  getEnvWithDefault
    (error $ "Environment variable " ++ name ++ " is undefined")
    name

Where error probably should be replaced by a more suitable error signalling mechanism.

[ndmitchell]

It's somewhat annoying - there is a perfect name for a partial version of getEnv and it is exactly getEnv just like in the base library. And there's a great name for getEnv too, which is lookupEnv.

However, for now I suggest just naming this one getEnvError - does that seem reasonable?

[kaoskorobase]

Why not follow Shake's convention of adding a tick to Action versions of library functions with the same name? So there could be getEnv', corresponding to System.Environment.getEnv and lookupEnv' corresponding to System.Environment.lookupEnv (aliased by getEnv). In a later release getEnv could just be removed.

[ndmitchell]

Shake has many conventions - way too many conventions. There's readFile' for the Action version of that, doesDirectoryExist for the Action version of that... It's way too inconsistent, and I'm not even sure what my preferred approach is.

[kaoskorobase]

Flip a coin? ;-)

Anyway, I'd be happy to see getEnvError in Shake regardless of what it's called! A minor convenience of the tick approach is that you can import Shake and base modules without qualifying imports.

[ndmitchell]

Rather than flip a coin I thought I'd solicit opinions: https://groups.google.com/d/msg/shake-build-system/8ZWQr2g3XM0/Z_hpQaEFAwAJ

Should that not work, I'll flip a coin :)