syntax: don't pick up chan chains (channel of channels)

[fatih]

Fixes the following wrong highlighting:

into:

This is a very weird error. We want to show an error if the chan
statement is in form of:

<- chan
chan <-

The correct form should be of course:

<-chan
chan<-

But for example, if we have a channel of channels (like below), the
selected piece of whitespace is also highlighted as an error

chan<- chan<-
      ^

Because it's the part of <- chan, whereas we should not treat as an
error because we have two chan<- and chan<- statements.

To fix this, we add a negative lookbehind regex statement. It will not
match a text if it's preceded with other text.

First let assemble the current regex for the case: <- chan
(receive-only annotation), which is:

\(<-\)\@<=\s\+\(chan\>\)\@=

The first part is:

\(<-\)\@<=\s\+\

Here the pattern @<= matches with zero width if the preceding atom
matches just before what follows. \s\+ means white space of at least
one character. So it matches whitespaces which precedes the
statement <-

Now the second part is only useful with the first part. I've added the
placeholder to make it more readable:

PLACEHOLDER(chan\>\)\@="

First the pattern \@= means to match the preceding atom with zero
width. So it will match the PLACEHOLDER till it finds a chan
statement. Once it finds it, it doesn't proceed anymore. So we only
highlight the whitespaces between <- and chan only if we have a
white space of at least one character

Finally we can introduce our fix now. We're going to pick the <-
statemetn if it doesn't follow a chan. So chan<- should NOT trigger
an error anymore, because we're not going to pick arrows in that form.

To match an arrow which doesn't follow a chan statement can be done via
a negative lookbehind, which is in the form of:

\(chan\)\@<!<-

This literally means: "Match <- only if it doesn't follow chan.
We're going to use this pattern for the plain <- pattern. Not we're
just going to replace this into our current regex:

This original regex:

\(<-\)\@<=\s\+\(chan\>\)\@=

will be changed into:

\(\(chan\)\@<!<-\)\@<=\s\+\(chan\>\)\@=

Fixes #655

[fxkr]

FTR: this introduces a regression; more details in my comment in #655

[fatih]

Hi @fxkr Thanks for testing it! I've force pushed the changes you mentioned. Can you please try again?

[fxkr]

• You may want to rebase it onto the current master.
• The commit message is outdated (specifically, the regex in the last line of the commit message.
• There's the extra f at the end of the last regex you touched, I'm sure that breaks something. Sorry that I didn't notice before.

-  syn match goSpaceError display "\(\(^\|[={(,;]\)\s*<-\)\@<=\s\+"
+  syn match goSpaceError display "\(\(^\|[={(,;]\)\s*<-\)\@<=\s\+f"

[fatih]

@fxkr fixed. Nice catch, seems I've accidentally added it.

[fatih]

Thanks @fxkr for the feedback. Please open a issue if you find other cases.