Commit 30a1a8c9 authored by Robert Griesemer's avatar Robert Griesemer

language for range clause

(I have deliberately left away the forms w/ := or = and
the forms with :)

R=r
DELTA=106  (44 added, 13 deleted, 49 changed)
OCL=21192
CL=21283
parent ef77c226
......@@ -28,7 +28,6 @@ Timeline (9/5/08):
Missing:
[ ] partial export of structs, methods
[ ] range statement: to be defined more reasonably
[ ] packages of multiple files
[ ] Helper syntax for composite types: allow names/indices for maps/arrays,
remove need for type in elements of composites
......@@ -56,7 +55,6 @@ Open issues:
[ ] old-style export decls (still needed, but ideally should go away)
[ ] like to have assert() in the language, w/ option to disable code gen for it
[ ] composite types should uniformly create an instance instead of a pointer
[ ] semantics of statements
[ ] need for type switch? (or use type guard with ok in tuple assignment?)
[ ] do we need anything on package vs file names?
[ ] type switch or some form of type test needed
......@@ -93,6 +91,8 @@ Decisions in need of integration into the doc:
Closed:
[x] semantics of statements - we just need to fill in the language, the semantics is mostly clear
[x] range statement: to be defined more reasonably
[x] need to be specific on (unsigned) integer operations: one must be able
to rely on wrap-around on overflow
[x] global var decls: "var a, b, c int = 0, 0, 0" is ok, but "var a, b, c = 0, 0, 0" is not
......@@ -198,7 +198,6 @@ Contents
If statements
Switch statements
For statements
Range statements
Go statements
Select statements
Return statements
......@@ -583,7 +582,9 @@ same identifier declared in an outer block.
3. The scope of a constant or variable extends textually from
after the declaration to the end of the innermost surrounding
block.
block. If the variable is declared in the init statement of an
if, for, or switch statement, the innermost surrounding block
is the block associated with the respective statement.
4. The scope of a parameter or result identifier is the body of the
corresponding function.
......@@ -2369,8 +2370,7 @@ Statements control execution.
Statement =
Declaration | LabelDecl | EmptyStat |
SimpleStat | GoStat | ReturnStat | BreakStat | ContinueStat | GotoStat |
FallthroughStat | Block | IfStat | SwitchStat | SelectStat | ForStat |
RangeStat .
FallthroughStat | Block | IfStat | SwitchStat | SelectStat | ForStat .
SimpleStat =
ExpressionStat | IncDecStat | Assignment | SimpleVarDecl .
......@@ -2589,68 +2589,99 @@ If the expression is omitted, it is equivalent to "true".
For statements
----
For statements are a combination of the "for" and "while" loops of C.
A for statement specifies repeated execution of a block. The iteration is
controlled by a condition, a for clause, or a range clause.
ForStat = "for" [ Condition | ForClause ] Block .
ForClause = [ InitStat ] ";" [ Condition ] ";" [ PostStat ] .
InitStat = SimpleStat .
ForStat = "for" [ Condition | ForClause | RangeClause ] Block .
Condition = Expression .
PostStat = SimpleStat .
A SimpleStat is a simple statement such as an assignment, a SimpleVarDecl,
or an increment or decrement statement. Therefore one may declare a loop
variable in the init statement.
for i := 0; i < 10; i++ {
print(i, "\n")
}
A for statement with just a condition executes until the condition becomes
false. Thus it is the same as C's while statement.
In its simplest form, a for statement specifies the repeated execution of
a block as long as a condition evaluates to true. The condition is evaluated
before each iteration. The type of the condition expression must be boolean.
If the condition is absent, it is equivalent to "true".
for a < b {
a *= 2
}
If the condition is absent, it is equivalent to "true".
for {
f()
}
A for statement with a for clause is also controlled by its condition, but
additionally it may specify an init and post statement, such as an assignment,
an increment or decrement statement. The init statement may also be a (simple)
variable declaration; no variables can be declared in the post statement.
Range statements
----
Range statements are a special control structure for iterating over
the contents of arrays and maps.
RangeStat = "range" IdentifierList ":=" RangeExpression Block .
RangeExpression = Expression .
A range expression must evaluate to an array, map or string. The identifier list must contain
either one or two identifiers. If the range expression is a map, a single identifier is declared
to range over the keys of the map; two identifiers range over the keys and corresponding
values. For arrays and strings, the behavior is analogous for integer indices (the keys) and
array elements (the values).
ForClause = [ InitStat ] ";" [ Condition ] ";" [ PostStat ] .
InitStat = SimpleStat .
PostStat = SimpleStat .
a := []int(1, 2, 3);
m := [string]map int("fo",2, "foo",3, "fooo",4)
For instance, one may declare an iteration variable in the init statement:
range i := a {
f(a[i]);
for i := 0; i < 10; i++ {
f(i)
}
range i, v := a {
f(v);
If present, the init statement is executed once before commencing the iteration;
the post statement is executed after each execution of the statement block (and
only if the block was executed). The scope of any variable declared in the init
statement ends with the end of the for statement block ($Declarations and scope
rules, Rule 3).
The init and post statement as well as the condition may be omitted; however
if either the init or post statement are present, the separating semicolons
must be present. If the condition is absent, it is equivalent to "true".
The following statements are equivalent:
for ; cond ; { S() } is the same as for cond { S() }
for true { S() } is the same as for { S() }
Alternatively, a for statement may be controlled by a range clause. A
range clause specifies iteration through all entries of an array or map.
For each entry it first assigns the current index or key to an iteration
variable - or the current (index, element) or (key, value) pair to a pair
of iteration variables - and then executes the block. Iteration terminates
when all entries have been processed, or if the for statement is terminated
early, for instance by a break or return statement.
RangeClause = IdentifierList ( "=" | ":=" ) "range" Expression .
The type of the right-hand expression in the range clause must be an array or
map, or a pointer to an array or map. If it is a pointer, it must not be nil.
The left-hand identifier list must contain one or two identifiers denoting the
iteration variables. The first variable is set to the current array index or
map key, and the second variable, if present, is set to the corresponding
array element or map value. The types of the array index (int) and element,
or of the map key and value respectively, must be assignment-compatible to
the iteration variables.
The iteration variables may be declared by the range clause (":="), in which
case their scope ends at the end of the for statement block ($Declarations and
scope rules, Rule 3). In this case their types are the array index and element,
or the map key and value types, respectively.
var a [10]string;
m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6};
for i, s := range a {
// type of i is int
// type of s is string
// s == a[i]
g(i, s)
}
range k, v := m {
assert(len(k) == v);
var key string;
var val interface {}; // value type of m is assignment-compatible to val
for key, value = range m {
h(key, value)
}
TODO: is this right?
// key == last map key encountered in iteration
// val == map[key]
If map entries that have not yet been processed are deleted during iteration,
they will not be processed. If map entries are inserted during iteration, the
behavior is implementation-dependent. Likewise, if the range expression is a
pointer variable, the behavior of assigning to that variable is implementation-
dependent. Assigning to the iteration variables during iteration simply changes
the values of those variables for the current iteration; it does not affect any
subsequent iterations.
Go statements
......
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