关于golang中的“ for”语法，在描述编译错误时是否使用了错误的术语？

Question

I tried to use something like

for i := 0; i < len(bytes); ++i {
        ...
}

It is not correct and I got an error

syntax error: unexpected ++, expecting expression

It was because of ++i is not an expression I thought.

Then I found out that i++ (it works in for loop) is not an expression as well according to the documentation.

Also I met that in some cases (now I think in all cases) a statement can not be used instead of expression.

Now if we come back to the error we see that for loop requires an expression. I was confused with that. I checked one more part of the documentation it turns out for requires a statement.

For statements with for clause

A "for" statement with a ForClause is also controlled by its condition, but additionally it may specify an init and a post statement

I started with question (which I liked more than the final question because it was about language non-acquaintance as I thought)

Is it special case for loop syntax that statement are accepted as expression or are there other cases in golang?

During writing the question and checking the documentation I end up to a questions

Is there used incorrect terminology in description of the error that should be fixed not to confuse? Or is it normally in some cases to substitute such terms as statement and expression?

Answer 1

The Go Programming Language Specification

Primary expressions

Primary expressions are the operands for unary and binary expressions.

PrimaryExpr =
  Operand |
  Conversion |
  PrimaryExpr Selector |
  PrimaryExpr Index |
  PrimaryExpr Slice |
  PrimaryExpr TypeAssertion |
  PrimaryExpr Arguments .

Selector       = "." identifier .
Index          = "[" Expression "]" .
Slice          = "[" [ Expression ] ":" [ Expression ] "]" |
                 "[" [ Expression ] ":" Expression ":" Expression "]" .
TypeAssertion  = "." "(" Type ")" .
Arguments      = "(" [ ( ExpressionList | Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" .

Operators and punctuation

The following character sequences represent operators:

++    
--    

Operators

Operators combine operands into expressions.

Expression = UnaryExpr | Expression binary_op Expression .
UnaryExpr  = PrimaryExpr | unary_op UnaryExpr .

binary_op  = "||" | "&&" | rel_op | add_op | mul_op .
rel_op     = "==" | "!=" | "<" | "<=" | ">" | ">=" .
add_op     = "+" | "-" | "|" | "^" .
mul_op     = "*" | "/" | "%" | "<<" | ">>" | "&" | "&^" .

unary_op   = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .

Operator precedence

The ++ and -- operators form statements, not expressions.

IncDec statements

The "++" and "--" statements increment or decrement their operands by the untyped constant 1. As with an assignment, the operand must be addressable or a map index expression.

IncDecStmt = Expression ( "++" | "--" ) .

---

++ and -- are operators. The ++ and -- operators form statements, not expressions.

IncDecStmt = Expression ( "++" | "--" ) . 

When the compiler encounters an ++ operator, it expects it to be immediately preceded by an expresssion.

For example,

package main

func main() {

    // syntax error: unexpected ++, expecting expression
    for i := 0; i < 1; ++i {}
}

Playground: https://play.golang.org/p/y2d9ijeMdw

Output:

main.go:6:21: syntax error: unexpected ++, expecting expression

The compiler complains about the syntax. It found a ++ operator without an immediately preceding expression: syntax error: unexpected ++, expecting expression.