mirror of
https://github.com/golang/go
synced 2024-11-22 01:24:42 -07:00
Explain goto constraints.
Add description of 'any'. Simplify and complete syntax for literals. SVN=114122
This commit is contained in:
Rob Pike
parent
42d7850608
commit
4c483aa159
108
doc/go_lang.txt
108
doc/go_lang.txt
@ -69,10 +69,9 @@ still under development.
|
||||
Typing, polymorphism, and object-orientation
|
||||
----
|
||||
|
||||
Go programs are strongly typed. Certain expressions, in particular map
|
||||
and channel accesses, can also be polymorphic. The language provides
|
||||
mechanisms to make use of such polymorphic values type-safe.
|
||||
|
||||
Go programs are strongly typed. Certain values can also be
|
||||
polymorphic. The language provides mechanisms to make use of such
|
||||
polymorphic values type-safe.
|
||||
|
||||
Interface types, building on structures with methods, provide
|
||||
the mechanisms to support object-oriented programming.
|
||||
@ -96,7 +95,7 @@ functions, structures, associated methods, and interfaces.
|
||||
|
||||
Go has no explicit notion of type parameters or templates. Instead,
|
||||
containers (such as stacks, lists, etc.) are implemented through the
|
||||
use of abstract data types operating on interface types.
|
||||
use of abstract operations on interface types or polymorphic values.
|
||||
|
||||
|
||||
Pointers and garbage collection
|
||||
@ -249,8 +248,8 @@ to refer to an arbitrary Unicode code point encoded in UTF-8.
|
||||
Digits and Letters
|
||||
----
|
||||
|
||||
octal_digit = { "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" } .
|
||||
decimal_digit = { "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" } .
|
||||
oct_digit = { "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" } .
|
||||
dec_digit = { "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" } .
|
||||
hex_digit = { "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" | "a" |
|
||||
"A" | "b" | "B" | "c" | "C" | "d" | "D" | "e" | "E" | "f" | "F" } .
|
||||
letter = "A" | "a" | ... "Z" | "z" | "_" .
|
||||
@ -265,7 +264,7 @@ Identifiers
|
||||
An identifier is a name for a program entity such as a variable, a
|
||||
type, a function, etc. An identifier must not be a reserved word.
|
||||
|
||||
identifier = letter { letter | decimal_digit } .
|
||||
identifier = letter { letter | dec_digit } .
|
||||
|
||||
a
|
||||
_x
|
||||
@ -317,6 +316,9 @@ caution.
|
||||
Two reserved words, "true" and "false", represent the
|
||||
corresponding boolean constant values.
|
||||
|
||||
There is also a polymorphic type, "any". The "any" type can represent
|
||||
a value of any type.
|
||||
|
||||
|
||||
Numeric literals
|
||||
----
|
||||
@ -343,14 +345,18 @@ instead of 1000000000).
|
||||
Floating point literals also represent an abstract, ideal floating
|
||||
point value that is constrained only upon assignment.
|
||||
|
||||
int_lit = [ "+" | "-" ] unsigned_int_lit .
|
||||
sign = "+" | "-" .
|
||||
int_lit = [ sign ] unsigned_int_lit .
|
||||
unsigned_int_lit = decimal_int_lit | octal_int_lit | hex_int_lit .
|
||||
decimal_int_lit = ( "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" )
|
||||
{ decimal_digit } .
|
||||
octal_int_lit = "0" { octal_digit } .
|
||||
{ dec_digit } .
|
||||
octal_int_lit = "0" { oct_digit } .
|
||||
hex_int_lit = "0" ( "x" | "X" ) hex_digit { hex_digit } .
|
||||
float_lit = [ "+" | "-" ] unsigned_float_lit .
|
||||
unsigned_float_lit = "the usual decimal-only floating point representation".
|
||||
float_lit = [ sign ] ( fractional_lit | exponential_lit ) .
|
||||
fractional_lit = { dec_digit } ( dec_digit "." | "." dec_digit )
|
||||
{ dec_digit } [ exponent ] .
|
||||
exponential_lit = dec_digit { dec_digit } exponent .
|
||||
exponent = ( "e" | "E" ) [ sign ] dec_digit { dec_digit }
|
||||
|
||||
07
|
||||
0xFF
|
||||
@ -399,7 +405,7 @@ The rules are:
|
||||
char_lit = "'" ( unicode_value | byte_value ) "'" .
|
||||
unicode_value = utf8_char | little_u_value | big_u_value | escaped_char .
|
||||
byte_value = octal_byte_value | hex_byte_value .
|
||||
octal_byte_value = "\" octal_digit octal_digit octal_digit .
|
||||
octal_byte_value = "\" oct_digit oct_digit oct_digit .
|
||||
hex_byte_value = "\" "x" hex_digit hex_digit .
|
||||
little_u_value = "\" "u" hex_digit hex_digit hex_digit hex_digit .
|
||||
big_u_value = "\" "U" hex_digit hex_digit hex_digit hex_digit
|
||||
@ -495,7 +501,6 @@ an error if placed in a character literal (it is not a single code
|
||||
point), and will appear as two code points if placed in a string
|
||||
literal.
|
||||
|
||||
|
||||
More about types
|
||||
----
|
||||
|
||||
@ -508,7 +513,7 @@ variable.
|
||||
|
||||
At any given time, a variable or value has exactly one dynamic
|
||||
type, which may be the same as the static type. (They will
|
||||
differ only if the variable has an interface type.)
|
||||
differ only if the variable has an interface type or "any" type.)
|
||||
|
||||
Compound types may be constructed from other types by
|
||||
assembling arrays, maps, channels, structures, and functions.
|
||||
@ -669,9 +674,7 @@ Channel types
|
||||
----
|
||||
|
||||
A channel provides a mechanism for two concurrently executing functions
|
||||
to exchange values and synchronize execution. A channel type can be
|
||||
'generic', permitting values of any type to be exchanged, or it may be
|
||||
'specific', permitting only values of an explicitly specified type.
|
||||
to synchronize execution and exchange values of a specified type.
|
||||
|
||||
Upon creation, a channel can be used both to send and to receive.
|
||||
By conversion or assignment, it may be restricted only to send or
|
||||
@ -838,9 +841,39 @@ and S1 and S2 also implement
|
||||
|
||||
they implement the Lock interface as well as the File interface.
|
||||
|
||||
It is legal to assign a pointer to a struct to a variable of
|
||||
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
|
||||
----
|
||||
|
||||
Given a variable of type "any", one can store any value into it by
|
||||
plain assignment or implicitly, such as through a function parameter
|
||||
or channel operation. Given an "any" variable v storing an underlying
|
||||
value of type T, one may:
|
||||
|
||||
- copy v's value to another variable of type "any"
|
||||
- extract the stored value by an explicit conversion operation T(v)
|
||||
- copy v's value to a variable of type T
|
||||
|
||||
Attempts to convert/extract to an incompatible type will yield nil.
|
||||
|
||||
No other operations are defined (yet).
|
||||
|
||||
Note that type
|
||||
interface {}
|
||||
is a special case that can match any struct type, while type
|
||||
any
|
||||
can match any type at all, including basic types, arrays, etc.
|
||||
|
||||
TODO: details about reflection
|
||||
|
||||
|
||||
Literals
|
||||
----
|
||||
|
||||
@ -1422,10 +1455,15 @@ array elements (the values).
|
||||
f(a[i]);
|
||||
}
|
||||
|
||||
range v, i := a {
|
||||
f(v);
|
||||
}
|
||||
|
||||
range k, v := m {
|
||||
assert(len(k) == v);
|
||||
}
|
||||
|
||||
TODO: is this right?
|
||||
|
||||
Break statements
|
||||
----
|
||||
@ -1457,16 +1495,6 @@ loop at the post statement.
|
||||
The optional identifier is analogous to that of a break statement.
|
||||
|
||||
|
||||
Goto statements
|
||||
----
|
||||
|
||||
A goto statement transfers control to the corresponding label statement.
|
||||
|
||||
GotoStat = "goto" identifier .
|
||||
|
||||
goto Error
|
||||
|
||||
|
||||
Label declaration
|
||||
----
|
||||
|
||||
@ -1476,9 +1504,25 @@ A label declaration serves as the target of a goto, break or continue statement.
|
||||
|
||||
Error:
|
||||
|
||||
TODO: what are the restrictions on the placement of labels
|
||||
and goto statements?
|
||||
|
||||
Goto statements
|
||||
----
|
||||
|
||||
A goto statement transfers control to the corresponding label statement.
|
||||
|
||||
GotoStat = "goto" identifier .
|
||||
|
||||
goto Error
|
||||
|
||||
Executing the goto statement must not cause any variables to come into
|
||||
scope that were not already in scope at the point of the goto. For
|
||||
instance, this example:
|
||||
|
||||
goto L; // BAD
|
||||
v := 3;
|
||||
L:
|
||||
|
||||
is erroneous because the jump to label L skips the creation of v.
|
||||
|
||||
Packages
|
||||
----
|
||||
@ -1549,3 +1593,5 @@ TODO
|
||||
- TODO: type switch?
|
||||
- TODO: select
|
||||
- TODO: words about slices
|
||||
- TODO: what is nil? do we type-test by a nil conversion or something else?
|
||||
|
||||
|
||||
Loading…
Reference in New Issue
Block a user