Multi handles inputs incorrectly

[snowleopard]

@rotsor spotted today that the implementation of multi always returns Just in the second case:

https://github.com/snowleopard/build/blob/master/src/Build/Multi.hs#L20

But what if we ask the resulting Tasks to build a list that contains input keys? We'll fetch them! This will lead to a cyclic dependency, breaking the build system.

The fix doesn't seem to be difficult: presumably, we need to apply tasks to all keys and if any of them returns Nothing we should also return Nothing. It's a bit awkward, but I don't see a better fix.

@ndmitchell Thoughts?

P.S.: Actually the correct check should probably be isInput = any (isNothing . tasks . partition) keys.

[Rotsor]

Thinking a bit more about this, maybe there's no cyclic dependency after all: [x;y] and [x] are distinct keys after all, so it seems ok for the former to depend on the latter.

On Tue, May 8, 2018, 00:02 Andrey Mokhov notifications@github.com wrote:

@Rotsor https://github.com/Rotsor spotted today that the implementation of multi always returns Just in the second case:

https://github.com/snowleopard/build/blob/master/src/Build/Multi.hs#L20

But what if we ask the resulting Tasks to build a list, which contains input keys? We'll fetch them! This will lead to a cyclic dependency, breaking the build system.

The fix doesn't seem to be difficult: presumably, we need to apply tasks to all keys and if any of them returns Nothing we should also return Nothing. It's a bit awkward, but I don't see a better fix.

@ndmitchell https://github.com/ndmitchell Thoughts?

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[snowleopard]

Oops. Indeed, looks fine in the morning :-)