strings: FieldFunc behaves differently from Split

[xformerfhs]

Go version

go version go1.24.1 windows/amd64

Output of go env in your module/workspace:

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set PKG_CONFIG=pkg-config

What did you do?

I wanted to split a string on several different characters, not only one, so I replaced a call of strings.Split(encodings, ":") by strings.FieldFunc(encodings, separatorFunc) where separatorFunc was this:

func separatorFunc(r rune) bool {
   return r == ':' || r == ','
}

What did you see happen?

With encodings := ":utf16bom" the calls had two different results:

Split: Slice with two elements [ "" "utf16bom" ]

FieldFunc: Slice with one element [ "utf16bom" ]

What did you expect to see?

With encodings := ":utf16bom" the calls should have delivered identical results:

Split: Slice with two elements [ "" "utf16bom" ]

FieldFunc: Slice with two elements [ "" "utf16bom" ]

[seankhliao]

If they were the same we wouldn't need 2 different functions.
I don;t think the behaviour can be changed, maybe Fields can also document that it collapses leading and trailing split characters.

[xformerfhs]

Well, no, the two functions would still be different. They would definitely not collapse into one.

The point is that Split only splits on one specific separator string, while FieldFunc allows multiple characters.

1. I - and quite a few other people out there - need a function that splits a string on any of different characters. In my example Split splits only on :, but I need a function that splits on either :, or ,. In general there should be a list of split characters or strings. There is no such function in the strings package and people recommend using FieldFunc for this, since there is no such thing as a SplitAny function analogous to the IndexAny function.

2. Split splits the string ":utf16bom" into two strings: An empty one and one with the value "utf16bom". This is what one would expect from a split function. If there is a separator and there are no characters before that separator the first element is an empty string. This is the correct way to do this. In my point of view it is a bug that FieldFunc ignores a leading separation character. What is the rationale behind this? This is totally unexpected and unexpected behaviour is never a good thing in programming. FieldFunc should return an empty string as the first element, if the scanned string begins with a rune where the separator function returns true.

[adonovan]

I agree with @seankhliao that the documentation of Fields and FieldsFunc could be more explicit about leading and trailing spaces, and that neither function can be changed at this point.

It sounds like you wish there was a third function in the library. Feel free to propose one.

[xformerfhs]

I agree with both of you that the best solution is to add a few words about this unexpected behaviour (i.e. bug) to the documentation.

As for the missing functions: I already wrote two functions SplitAny and CountAny along the lines of IndexAny. They can easily be adapted to more general function, but they do for me now.

[gopherbot]

Change https://go.dev/cl/658218 mentions this issue: bytes,strings: document Fields trimming of leading and trailing characters