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