Better line indication where compiler found a channel push syntax error

[jorgeluismireles]

Describe the feature

In Golang the channel push statement <- ch is valid. In V is mandatory to have _ := <-ch but unexpectedly this also is accepted in V:

_ :=


<-

ch

This assignation valid after several rows and spaces creates problems with the line the compiler indicates as error.

Use Case

Next program has a V syntax error in line 20 <-ch and the compiler notifies the error far before in line 12 thinking there is the receiver of invalid type:

fn main() {
	result := chan string{}
	spawn fn(result chan string){
		// ...
	}(result)
	spawn start(result)	
}

fn start(result chan string) {
	ok := chan bool{}
	spawn read(result, ok)
	write(5) or { // compiler says error is here
		// some
		// extra
		// comments
		// or code
		result<- 'err:${err}'
		ok<- false
	}
	<-ok // syntax error is here
}

fn read(result chan string, ok chan bool) {
	// ...
}

fn write(n int) ! {
	// ...
}

Produces

code.v:12:2: error: cannot push on non-channel `void`
   10 |     ok := chan bool{}
   11 |     spawn read(result, ok)
   12 |     write(5) or {
      |     ~~~~~~~~
   13 |         // bla
   14 |         // bla
Exited with error status 1

It takes time to realize where the problem really is.

Proposed Solution

Improve the error line like this, showing the line of the <- operator.

code.v:20:2: error: cannot push on non-channel `void`
   19 |     }
   20 |     <-ok
      |     ~~~~~
   21 |}

Then the fix is done in line 20:

_ := <-ok

Other features to consider:

Accept as valid (as in Golang)

<- ch

Reject as invalid having the receiver and producer in different lines:

_ := 


<-

 ch

Other Information

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

Version used

V 0.4.9 991ec1f

Environment details (OS name and version, etc.)

https://play.vlang.io/p/e14013a25d

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[huly-for-github]

Connected to Huly®: V_0.6-21997