Avoid Data.List.{head,tail}

[Bodigrim]

CLC has approved the proposal to add {-# WARNING #-} to Data.List.{head,tail} (haskell/core-libraries-committee#87). It means that usage of head and tail will emit compile-time warnings.

This patch eliminates the only usage of head in unix.

[hasufell]

This changes behaviour though right? Passing in an empty dir wasn't possible before this patch?

[Bodigrim]

@hasufell good point. I filed #260 to track the issue with mdir = Just "", but scaled back this PR to preserve the existing (partial) semantics, making the very minimal change just to avoid head and tail. How does it look now?

[hasufell]

Sure, but why does head cause a warning and last doesn't?

[Bodigrim]

The CLC proposal asked to add {-# WARNING #-} for head and tail only, but not for last and init. The rationale behind that was that it is considerably more difficult to migrate away from last and init: one cannot easily replace them with pattern matching and there is no unsnoc :: [a] -> Maybe ([a], a) available.

[hasufell]

The CLC proposal asked to add {-# WARNING #-} for head and tail only, but not for last and init. The rationale behind that was that it is considerably more difficult to migrate away from last and init: one cannot easily replace them with pattern matching and there is no unsnoc :: [a] -> Maybe ([a], a) available.

I wouldn't mind if we inline unsnoc internally until it hits base.

https://hackage.haskell.org/package/extra-1.7.12/docs/src/Data.List.Extra.html#unsnoc

[Bodigrim]

@hasufell good to go? I replaced last with our own unsnoc.