On Fri, Sep 16, 2016 at 3:42 AM, Jan Mercl <0xj...@gmail.com> wrote: > Consider this program (https://play.golang.org/p/V0fu9rD8_D) > > package main > > import ( > "fmt" > ) > > func main() { > c := make(chan int, 1) > c <- 1 > d := <-chan int(c) > fmt.Printf("%T\n", d) > e := (<-chan int)(c) > fmt.Printf("%T\n", e) > } > > Its output is > > int > <-chan int > > Intuitively it seems ok (and I believe the output is correct). Let's look > closer on the RHS expression of line > > d := <-chan int(c) > > From the specs: > > Expression = UnaryExpr | Expression binary_op Expression . > UnaryExpr = PrimaryExpr | unary_op UnaryExpr . > unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" . > PrimaryExpr = Operand | Conversion | PrimaryExpr Selector | PrimaryExpr > Index | PrimaryExpr Slice | PrimaryExpr TypeAssertion | PrimaryExpr > Arguments . > Conversion = Type "(" Expression [ "," ] ")" . > Type = TypeName | TypeLit | "(" Type ")" . > TypeLit = ArrayType | StructType | PointerType | FunctionType | > InterfaceType | SliceType | MapType | ChannelType . > ChannelType = ( "chan" | "chan" "<-" | "<-" "chan" ) ElementType . > > It seems to me that parsing `<-chan int(c)` as > > a receive operation from a conversion of `c` to type `chan int` > > is as valid as parsing it as > > a conversion of `c` to type `<-chan int`. > > In the later case the program would output > > <-chan int > <-chan int > > I must be missing something. Which rule selects the first parse? Can anybody > please enlighten me? Thanks in advance.
The rule you are looking for is at https://golang.org/ref/spec#Conversions : "If the type starts with the operator * or <-, or if the type starts with the keyword func and has no result list, it must be parenthesized when necessary to avoid ambiguity." Ian -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
