Commit f4f58837 authored by Rob Pike's avatar Rob Pike

Clarify conversions, include making them constructors for arrays etc.

SVN=118194
parent 9bc7b08a
......@@ -623,54 +623,6 @@ followed by a parenthesized expression list. In effect, they are a
conversion from expression list to compound value.
Array literals
----
Array literals represent array values. All the contained expressions must
be of the same type, which is the element type of the resulting array.
ArrayLit = ArrayType "(" [ ExpressionList ] ")" .
[]int()
[]int(1, 2, 3)
[]string("x", "y")
Unresolved issues: Are elements converted? What about length?
Map Literals
----
Map literals represent map values. They comprise a list of (key, value)
pairs written as successive values.
All keys must have the same type; all values must have the same type.
MapLit = MapType "(" KeyValueList ")" .
KeyValueList = [ KeyValue { "," KeyValue } ].
KeyValue = Expression "," Expression .
[string]map int("one",1, "two",2)
[int]map bool(2,true, 3,true, 5,true, 7,true)
Unresolved issues: Are elements converted?
Colon for a separator or comma?
Struct literals
----
Struct literals represent struct constants. They comprise a list of
expressions that represent the individual fields of a struct. The
individual expressions must match those of the specified struct type.
StructLit = StructType "(" [ ExpressionList ] ")" .
Point(2, 3)
ColoredPoint(4, 4, "green")
struct { a, b int } (7, 8)
Pointer types
----
......@@ -699,8 +651,6 @@ can be constructed such as:
By the end of the package source, all forward-declared types must be
fully declared if they are used.
There are no pointer literals.
Channel types
----
......@@ -727,9 +677,6 @@ particular to dereference a channel pointer.
var ch *chan int;
ch = new(chan int); // new returns type *chan int
There are no channel literals.
Function types
----
......@@ -879,8 +826,6 @@ compatible interface type. It is legal to assign an interface
variable to any struct pointer variable but if the struct type is
incompatible the result will be nil.
There are no interface literals.
The polymorphic "any" type
----
......@@ -949,9 +894,7 @@ vice versa. Note that the declaration order of the methods is not relevant.
Literals
----
Literal = BasicLit | CompoundLit .
BasicLit = char_lit | string_lit | int_lit | float_lit | "nil" .
CompoundLit = ArrayLit | MapLit | StructLit | FunctionLit .
Literal = char_lit | string_lit | int_lit | float_lit | FunctionLit | "nil" .
Declarations
......@@ -1159,8 +1102,9 @@ Expression syntax is based on that of C but with fewer precedence levels.
Expression "." "(" Type ")" .
Call = Expression "(" [ ExpressionList ] ")" .
Conversion = TypeName "(" Expression ")" |
"convert" "(" Type "," Expression ")" .
Conversion = "convert" "(" Type [ "," ExpressionList ] ")" |
ConversionType "(" [ ExpressionList ] ")" .
ConversionType = TypeName | ArrayType | MapType | StructType | InterfaceType .
Allocation = "new" "(" Type [ "," Expression ] ")" .
binary_op = log_op | rel_op | add_op | mul_op .
......@@ -1245,7 +1189,7 @@ The keyword
represents the ``zero'' value for a pointer type or interface type.
The only operations allowed for nil are to assign it to a pointer or
interface value and to compare it for equality or inquality with a
interface variable and to compare it for equality or inequality with a
pointer or interface value.
var p *int;
......@@ -1261,6 +1205,8 @@ TODO: how does this definition jibe with using nil to specify
conversion failure if the result is not of pointer type, such
as an any variable holding an int?
TODO: if interfaces were explicitly pointers, this gets simpler.
Allocation
----
......@@ -1294,40 +1240,82 @@ TODO: argument order for dimensions in multidimensional arrays
Conversions
----
There are three ways to convert a value from one type to another.
The most general is a call to the intrinsic special function "convert"
with arguments the type name and the value to be converted.
var i int = convert(int, PI * 1000.0);
chars_as_ints := convert([]int, "now is the time");
If the destination type is a known type name, the conversion can be
rewritten to look syntactically like a call to a function with that
name.
i := int(PI * 1000.0);
s := AStructType(an_interface_variable);
A conversion can be written as a parenthesized type after a period.
Although intended for ease of conversion within a method call chain,
this form works in any expression context.
Conversions create new values of a specified type derived from the
elements of a list of expressions of a different type.
The most general conversion takes the form of a call to "convert",
with the result type and a list of expressions as arguments:
convert(int, PI * 1000.0);
convert([]int, 1, 2, 3, 4);
If the result type is a basic type, pointer type, or
interface type, there must be exactly one expression and there is a
specific set of permitted conversions, detailed later in the section.
These conversions are called ``simple conversions''.
TODO: if interfaces were explicitly pointers, this gets simpler.
convert(int, 3.14159);
convert(uint32, ~0);
convert(interface{}, new(S))
convert(*AStructType, interface_value)
For other result types - arrays, maps, structs - the expressions
form a list of values to be assigned to successive elements of the
resulting value. If the type is an array or map, the list may even be
empty. Unlike in a simple conversion, the types of the expressions
must be equivalent to the types of the elements of the result type;
the individual values are not converted. For instance, if result
type is []int, the expressions must be all of type int, not float or
uint. (For maps, the successive elements must be key-value pairs).
For arrays and struct types, if fewer elements are provided than
specified by the result type, the missing elements are
initialized to the respective ``zero'' value for that element type.
These conversions are called ``compound conversions''.
convert([]int) // empty array of ints
convert([]int, 1, 2, 3)
convert([5]int, 1, 2); // == convert([5]int, 1, 2, 0, 0, 0)
convert(map[string]int, "1", 1, "2", 2)
convert(struct{ x int; y float }, 3, sqrt(2.0))
There is syntactic help to make conversion expressions simpler to write.
If the result type is of ConversionType (a type name, array type,
map type, structure type, or interface type, essentially anything
except a pointer), the conversion can be rewritten to look
syntactically like a call to a function whose name is the type:
int(PI * 1000.0);
AStructType(an_interface_variable);
struct{ x int, y float }(3, sqrt(2.0))
[]int(1, 2, 3, 4);
map[string]int("1", 1, "2", 2);
This notation is convenient for declaring and initializing
variables of composite type:
primes := []int(2, 3, 5, 7, 9, 11, 13);
Simple conversions can also be written as a parenthesized type after
an expression and a period. Although intended for ease of conversion
within a method call chain, this form works in any expression context.
TODO: should it?
var s *AStructType = vec.index(2).(*AStructType);
fld := vec.index(2).(*AStructType).field;
f := 1000.(float);
a := foo[i].(string);
TODO: are there parameters to any conversions? go.y has oexpr_list as the
contents of a TypeName() conversion; i expected expr instead and that's what
the others have.
Only some conversions are permitted.
As said, for compound conversions the element types must be equivalent.
For simple conversions, the types can differ but only some combinations
are permitted:
1) Between integer types. If the value is a signed quantity, it is
sign extended to implicit infinite precision; otherwise it is zero
extended. It is then truncated to fit in the destination type size.
extended. It is then truncated to fit in the result type size.
For example, uint32(int8(0xFF)) is 0xFFFFFFFF. The conversion always
yields a valid value; for instance, there is no signal for overflow.
yields a valid value; there is no signal for overflow.
2) Between integer and floating point types, or between floating point
types. To avoid overdefining the properties of the conversion, for
......@@ -1344,6 +1332,18 @@ should incompatible conversions fail immediately?
conversions yield nil values. TODO: is nil right here? Or should
incompatible conversions fail immediately?
5) Strings permit two special conversions.
5a) Converting an integer value yields a string containing the UTF-8
representation of the integer.
string(0x65e5) // "\u65e5"
5b) Converting an array of uint8s yields a string whose successive
bytes are those of the array. (Recall byte is a synonym for uint8.)
string([]byte('h', 'e', 'l', 'l', 'o')) // "hello"
Note that there is no linguistic mechanism to convert between pointers
and integers. A library may be provided under restricted circumstances
to acccess this conversion in low-level code but it will not be available
......
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