reuse fmap's input list, if output type is the same as the input type

[awalterschulze]

[awalterschulze]

This is a performance improvement.

[rogpeppe]

I'm not sure that this is necessarily a good thing - there are arguments both ways. It's not always OK to modify the slice that's being passed in. I suspect that it might be better for there to be a different plugin that modifies in place.

[awalterschulze]

This is why I would love to have immutability in Go, so that we don't have to make this decision, but now here we are.

[awalterschulze]

So you would prefer to rather have an inplace variant of fmap and filter than it being the default?