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- documenting old "new()"

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- adding "init()"
- fixing some bugs with slice documentation

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Robert Griesemer 2009-01-06 13:23:20 -08:00
parent af0143ce03
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doc/go_spec.txt
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@ -3,7 +3,7 @@ The Go Programming Language Specification (DRAFT)
Robert Griesemer, Rob Pike, Ken Thompson
(January 5, 2009)
(January 6, 2009)
----
@ -224,6 +224,7 @@ Contents
Length and capacity
Conversions
Allocation
Making slices, maps, and channels
Packages
@ -749,7 +750,7 @@ The predeclared constants:
The predeclared functions (note: this list is likely to change):
cap(), convert(), len(), new(), panic(), panicln(), print(), println(), typeof(), ...
cap(), convert(), len(), make(), new(), panic(), panicln(), print(), println(), typeof(), ...
Exported declarations
@ -1451,7 +1452,7 @@ if the static type of the value implements the interface or if the value is "nil
Slice types
----
An (array) slice type denotes the set of all slices (segments) of arrays
A slice type denotes the set of all slices (segments) of arrays
(§Array types) of a given element type, and the value "nil".
The number of elements of a slice is called its length; it is never negative.
The elements of a slice are designated by indices which are
@ -1478,12 +1479,21 @@ and the following relationship between "len()" and "cap()" holds:
0 <= len(a) <= cap(a)
The value of an uninitialized slice is "nil", and its length and capacity
are 0. A new, initialized slice value for a given elemen type T is
created using the built-in function "new", which takes a slice type
are 0. A new, initialized slice value for a given element type T is
made using the built-in function "make", which takes a slice type
and parameters specifying the length and optionally the capacity:
new([]T, length)
new([]T, length, capacity)
make([]T, length)
make([]T, length, capacity)
The "make()" call allocates a new underlying array to which the returned
slice value refers. More precisely, calling "make"
make([]T, length, capacity)
is effectively the same as allocating an array and slicing it
new([capacity]T)[0 : length]
Assignment compatibility: Slices are assignment compatible to variables
of the same type.
@ -1506,7 +1516,8 @@ This creates the sub-slice consisting of the elements "a[i]" through "a[j - 1]"
"i <= j <= cap(a)". The length of the new slice is "j - i". The capacity of
the slice is "cap(a) - i"; thus if "i" is 0, the slice capacity does not change
as a result of a slice operation. The type of a sub-slice is the same as the
type of the slice.
type of the slice. Unlike the capacity, the length of a sub-slice
may be larger than the length of the original slice.
TODO what are the proper restrictions on slices?
TODO describe equality checking against nil
@ -1536,11 +1547,11 @@ The length of a map "m" can be discovered using the built-in function
len(m)
The value of an uninitialized map is "nil". A new, initialized map
value for given key and value types K and V is created using the built-in
function "new" which takes the map type and an (optional) capacity as arguments:
The value of an uninitialized map is "nil". A new, empty map
value for given key and value types K and V is made using the built-in
function "make" which takes the map type and an (optional) capacity as arguments:
my_map := new(map[K] V, 100);
my_map := make(map[K] V, 100);
The map capacity is an allocation hint for more efficient incremental growth
of the map.
@ -1573,10 +1584,10 @@ bi-directional (unconstrained), send, or receive.
<-chan int // can only receive ints
The value of an uninitialized channel is "nil". A new, initialized channel
value for a given element type T is created using the built-in function "new",
which takes the channel type and an (optional) capacity as arguments:
value for a given element type T is made using the built-in function "make",
which takes the channel type and an optional capacity as arguments:
my_chan = new(chan int, 100);
my_chan = make(chan int, 100);
The capacity sets the size of the buffer in the communication channel. If the
capacity is greater than zero, the channel is asynchronous and, provided the
@ -1978,29 +1989,25 @@ TODO: Need to expand map rules for assignments of the form v, ok = m[k].
Slices
----
Strings and arrays can be ``sliced'' to construct substrings or subarrays.
The index expressions in the slice select which elements appear in the
result. The result has indexes starting at 0 and length equal to the difference
in the index values in the slice. After
Strings, arrays, and slices can be ``sliced'' to construct substrings or descriptors
of subarrays. The index expressions in the slice select which elements appear
in the result. The result has indexes starting at 0 and length equal to the
difference in the index values in the slice. After slicing the array "a"
a := []int(1,2,3,4)
slice := a[1:3]
a := [4]int{1, 2, 3, 4};
s := a[1:3];
The array ``slice'' has length two and elements
the slice "s" has type "[]int", length 2, and elements
slice[0] == 2
slice[1] == 3
s[0] == 2
s[1] == 3
The index values in the slice must be in bounds for the original
array (or string) and the slice length must be non-negative.
Slices are new arrays (or strings) storing copies of the elements, so
changes to the elements of the slice do not affect the original.
In the example, a subsequent assignment to element 0,
slice[0] = 5
would have no effect on ``a''.
If the sliced operand is a string, the result of the slice operation is another
string (§String types). If the sliced operand is an array or slice, the result
of the slice operation is a slice (§Slice types).
Type guards
@ -2408,15 +2415,15 @@ section describes their form and function.
Here the term "channel" means "variable of type chan".
A channel is created by allocating it:
The built-in function "make" makes a new channel value:
ch := new(chan int)
ch := make(chan int)
An optional argument to new() specifies a buffer size for an
An optional argument to "make()" specifies a buffer size for an
asynchronous channel; if absent or zero, the channel is synchronous:
sync_chan := new(chan int)
buffered_chan := new(chan int, 10)
sync_chan := make(chan int)
buffered_chan := make(chan int, 10)
The send operation uses the binary operator "<-", which operates on
a channel and a value (expression):
@ -3083,9 +3090,12 @@ Predeclared functions
cap
convert
len
make
new
panic
panicln
print
println
typeof
@ -3097,20 +3107,28 @@ this is a good idea).
Length and capacity
----
The predeclared function "len()" takes a value of type string,
array or map type, or of pointer to array or map type, and
returns the length of the string in bytes, or the number of array
of map elements, respectively.
Call Argument type Result
The predeclared function "cap()" takes a value of array or pointer
to array type and returns the number of elements for which there
is space allocated in the array. For an array "a", at any time the
following relationship holds:
len(s) string, *string string length (in bytes)
[n]T, *[n]T array length (== n)
[]T, *[]T slice length
map[K]T, *map[K]T map length
chan T number of elements in channel buffer
0 <= len(a) <= cap(a)
cap(s) []T, *[]T capacity of s
map[K]T, *map[K]T capacity of s
chan T channel buffer capacity
TODO(gri) Change this and the following sections to use a table indexed
by functions and parameter types instead of lots of prose.
TODO: confirm len() and cap() for channels
The type of the result is always "int" and the implementation guarantees that
the result always fits into an "int".
The capacity of a slice or map is the number of elements for which there is
space allocated in the underlying array (for a slice) or map. For a slice "s",
at any time the following relationship holds:
0 <= len(s) <= cap(s)
Conversions
@ -3161,31 +3179,51 @@ have to be written as type guards? (§Type guards)
Allocation
----
The built-in function "new()" takes a type "T", optionally followed by a
type-specific list of expressions. It returns a value of type "T" (possibly
by allocating memory in the heap).
TODO describe initialization
The built-in function "new" takes a type "T" and returns a value of type "*T".
The memory is initialized as described in the section on initial values
(§Program initialization and execution).
new(type [, optional list of expressions])
new(T)
For instance
type S struct { a int; b float }
new(*S)
new(S)
dynamically allocates memory for a variable of type S, initializes it
(a=0, b=0.0), and returns a value of type *S pointing to that variable.
The only defined parameters affect sizes for allocating arrays,
buffered channels, and maps.
s := new([]int); # slice
c := new(chan int, 10); # channel with a buffer size of 10
m := new(map[string] int, 100); # map with initial space for 100 elements
TODO Once this has become clearer, connect new() and make() (new() may be
explained by make() and vice versa).
TODO revisit this section
Making slices, maps, and channels
----
The built-in function "make" takes a type "T", optionally followed by a
type-specific list of expressions. It returns a value of type "T". "T"
must be a slice, map, or channel type.
The memory is initialized as described in the section on initial values
(§Program initialization and execution).
make(T [, optional list of expressions])
For instance
make(map[string] int)
creates a new map value and initializes it to an empty map.
The only defined parameters affect sizes for allocating slices, maps, and
buffered channels:
s := make([]int, 10, 100); # slice with len(s) == 10, cap(s) == 100
c := make(chan int, 10); # channel with a buffer size of 10
m := make(map[string] int, 100); # map with initial space for 100 elements
TODO Once this has become clearer, connect new() and make() (new() may be
explained by make() and vice versa).
----
@ -3274,12 +3312,12 @@ Here is a complete example Go package that implements a concurrent prime sieve:
// The prime sieve: Daisy-chain Filter processes together.
func Sieve() {
ch := new(chan int); // Create a new channel.
ch := make(chan int); // Create a new channel.
go Generate(ch); // Start Generate() as a subprocess.
for {
prime := <-ch;
print(prime, "\n");
ch1 := new(chan int);
ch1 := make(chan int);
go Filter(ch, ch1, prime);
ch = ch1
}