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- exceptional conditions during expression evaluation are undefined

- "nil" for interfaces, comparison against "nil"

R=r
DELTA=38  (24 added, 5 deleted, 9 changed)
OCL=16207
CL=16211
This commit is contained in:
Robert Griesemer 2008-09-30 13:02:50 -07:00
parent 4d571c9093
commit 52a548034e

View File

@ -47,11 +47,9 @@ Open issues according to gri:
(issue: what happens in len() + const - what is the type?)
[ ] Do composite literals create a new literal each time (gri thinks yes)
[ ] consider syntactic notation for composite literals to make them parseable w/o type information
[ ] nil and interfaces - can we test for nil, what does it mean, etc.
[ ] type switch or some form of type test needed
[ ] what is the meaning of typeof()
[ ] at the moment: type T S; strips any methods of S. It probably shouldn't.
[ ] talk about underflow/overflow of 2's complement numbers (defined vs not defined).
[ ] 6g allows: interface { f F } where F is a function type. fine, but then we should
also allow: func f F {}, where F is a function type.
[ ] provide composite literal notation to address array indices: []int{ 0: x1, 1: x2, ... }
@ -60,6 +58,8 @@ Decisions in need of integration into the doc:
[ ] pair assignment is required to get map, and receive ok.
Closed issues:
[x] nil and interfaces - can we test for nil, what does it mean, etc.
[x] talk about underflow/overflow of 2's complement numbers (defined vs not defined).
[x] change wording on array composite literals: the types are always fixed arrays
for array composites
[x] meaning of nil
@ -1249,6 +1249,11 @@ TODO(gri) This may be overly constraining. What about "len(a) + c" where
c is an ideal number? Is len(a) of type int, or of an ideal number? Probably
should be ideal number, because for fixed arrays, it is a constant.
If an exceptional condition occurs during the evaluation of an expression
(that is, if the result is not mathematically defined or not in the range
of representable values for its type), the behavior is undefined. For
instance, the behavior of integer under- or overflow is not defined.
Operands
----
@ -1511,9 +1516,6 @@ Operators combine operands into expressions.
unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
TODO: If we allow non-blocking sends only in the form "ok = ch <- x", it doesn't
make sense to give binary "<-" precedence 3. It should be at the lowest level. TBD.
The operand types in binary operations must be equal, with the following exceptions:
- The right operand in a shift operation must be
@ -1625,7 +1627,8 @@ Comparison operators
Comparison operators yield a boolean result. All comparison operators apply
to strings and numeric types. The operators "==" and "!=" also apply to
boolean values and to pointer types (including the value "nil").
boolean values and to pointer types (including the value "nil"). Finally,
"==" and "!=" can also be used to compare interface types against "nil".
== equal
!= not equal
@ -1634,7 +1637,12 @@ boolean values and to pointer types (including the value "nil").
> greater
>= greater or equal
TODO: Can we/should we be able to compare interfaces?
Strings are compared byte-wise (lexically).
Interfaces can be tested against "nil" (§Interface types).
For a value "v" of interface type, "v == nil" is true only if the predeclared
constant "nil" is assigned explicitly to "v" (§Assignments), or "v" has not
been modified since creation (§Program initialization and execution).
Logical operators
@ -1845,11 +1853,22 @@ Expression statements
IncDec statements
----
The "++" and "--" statements increment or decrement their operands
by the (ideal) constant value 1.
IncDecStat = Expression ( "++" | "--" ) .
The following assignment statements (§Assignments) are semantically
equivalent:
a[i]++
IncDec statement Assignment
x++ x += 1
x-- x -= 1
Note that ++ and -- are not operators for expressions.
Both operators apply to integer and floating point types only.
Note that increment and decrement are statements, not expressions.
For instance, "x++" cannot be used as an operand in an expression.
Assignments
@ -2522,12 +2541,12 @@ Program initialization and execution
----
When memory is allocated to store a value, either through a declaration
or new(), and no explicit initialization is provided, the memory is
or "new()", and no explicit initialization is provided, the memory is
given a default initialization. Each element of such a value is
set to the ``zero'' for that type: "false" for booleans, "0" for integers,
"0.0" for floats, '''' for strings, and nil for pointers. This intialization
is done recursively, so for instance each element of an array of integers will
be set to 0 if no other value is specified.
"0.0" for floats, '''' for strings, and "nil" for pointers and interfaces.
This intialization is done recursively, so for instance each element of an
array of integers will be set to 0 if no other value is specified.
These two simple declarations are equivalent: