Merge pull request #2017 from xushiwei/q

mini spec: Order of evaluation

Author: xushiwei

doc/spec-mini.md

@@ -836,6 +836,43 @@ var a map[string]any = {"Monday": 1, "Sunday": 7}
 echo a
 ```
 
+### Order of evaluation
+
+At package level, [initialization dependencies]() determine the evaluation order of individual initialization expressions in [variable declarations](). Otherwise, when evaluating the [operands]() of an expression, assignment, or [return statement](#return-statements), all function calls, method calls, [receive operations](), and [binary logical operations](#logical-operators) are evaluated in lexical left-to-right order.
+
+For example, in the (function-local) assignment
+
+```go
+y[f()], ok = g(z || h(), i()+x[j()], <-c), k()
+```
+
+the function calls and communication happen in the order `f()`, `h()` (if z evaluates to false), `i()`, `j()`, `<-c`, `g()`, and `k()`. However, the order of those events compared to the evaluation and indexing of `x` and the evaluation of `y` and `z` is not specified, except as required lexically. For instance, `g` cannot be called before its arguments are evaluated.
+
+```go
+a := 1
+f := func() int { a++; return a }
+x := [a, f()]      // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified
+m := {a: 1, a: 2}  // m may be {2: 1} or {2: 2}: evaluation order between the two map assignments is not specified
+n := {a: f()}      // n may be {2: 3} or {3: 3}: evaluation order between the key and the value is not specified
+```
+
+At package level, initialization dependencies override the left-to-right rule for individual initialization expressions, but not for operands within each expression:
+
+```go
+var a, b, c = f() + v(), g(), sqr(u()) + v()
+
+func f() int        { return c }
+func g() int        { return a }
+func sqr(x int) int { return x*x }
+
+// functions u and v are independent of all other variables and functions
+```
+
+The function calls happen in the order `u()`, `sqr()`, `v()`, `f()`, `v()`, and `g()`.
+
+Floating-point operations within a single expression are evaluated according to the associativity of the operators. Explicit parentheses affect the evaluation by overriding the default associativity. In the expression `x + (y + z)` the addition `y + z` is performed before adding `x`.
+
+
 ## Statements
 
 Statements control execution.