Commit b40831b1 authored by griesemer's avatar griesemer Committed by Robert Griesemer

spec: explicitly define notion of "representability" (clarification)

Throughout the spec we use the notion of a constant x being
representable by a value of type T. While intuitively clear,
at least for floating-point and complex constants types, the
concept was not well-defined. In the section on Conversions
there was an extra rule for floating-point types only and it
missed the case of floating-point values overflowing to an
infinity after rounding.

Since the concept is important to Go, and a compiler most
certainly will have a function to test "representability",
it seems warranted to define the term explicitly in the spec.

This change introduces a new entry "Representability" under
the section on "Properties of types and values", and defines
the term explicitly, together with examples.

The phrase used is "representable by" rather than "representable as"
because the former use is prevalent in the spec.

Additionally, it clarifies that a floating-point constant
that overflows to an infinity after rounding is never
representable by a value of a floating-point type, even though
infinities are valid values of IEEE floating point types.
This is required because there are not infinite value constants
in the language (like there is also no -0.0) and representability
also matters for constant conversions. This is not a language
change, and type-checkers have been following this rule before.

The change also introduces links throughout the spec to the new
section as appropriate and removes duplicate text and examples
elsewhere (Constants and Conversions sections), leading to
simplifications in the relevant paragraphs.

Fixes #15389.

Change-Id: I8be0e071552df0f18998ef4c5ef521f64ffe8c44
Reviewed-on: https://go-review.googlesource.com/57530Reviewed-by: default avatarRob Pike <r@golang.org>
Reviewed-by: default avatarIan Lance Taylor <iant@golang.org>
Reviewed-by: default avatarMatthew Dempsky <mdempsky@google.com>
parent 25bbb695
......@@ -577,11 +577,7 @@ or <a href="#Conversions">conversion</a>, or implicitly when used in a
<a href="#Assignments">assignment</a> or as an
operand in an <a href="#Expressions">expression</a>.
It is an error if the constant value
cannot be represented as a value of the respective type.
For instance, <code>3.0</code> can be given any integer or any
floating-point type, while <code>2147483648.0</code> (equal to <code>1&lt;&lt;31</code>)
can be given the types <code>float32</code>, <code>float64</code>, or <code>uint32</code> but
not <code>int32</code> or <code>string</code>.
cannot be <a href="#Representability">represented</a> as a value of the respective type.
</p>
<p>
......@@ -861,7 +857,8 @@ ElementType = Type .
<p>
The length is part of the array's type; it must evaluate to a
non-negative <a href="#Constants">constant</a> representable by a value
non-negative <a href="#Constants">constant</a>
<a href="#Representability">representable</a> by a value
of type <code>int</code>.
The length of array <code>a</code> can be discovered
using the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
......@@ -1514,7 +1511,7 @@ are different because <code>B0</code> is different from <code>[]string</code>.
<p>
A value <code>x</code> is <i>assignable</i> to a <a href="#Variables">variable</a> of type <code>T</code>
("<code>x</code> is assignable to <code>T</code>") in any of these cases:
("<code>x</code> is assignable to <code>T</code>") if one of the following conditions applies:
</p>
<ul>
......@@ -1540,12 +1537,68 @@ and at least one of <code>V</code> or <code>T</code> is not a defined type.
is a pointer, function, slice, map, channel, or interface type.
</li>
<li>
<code>x</code> is an untyped <a href="#Constants">constant</a> representable
<code>x</code> is an untyped <a href="#Constants">constant</a>
<a href="#Representability">representable</a>
by a value of type <code>T</code>.
</li>
</ul>
<h3 id="Representability">Representability</h3>
<p>
A <a href="#Constants">constant</a> <code>x</code> is <i>representable</i>
by a value of type <code>T</code> if one of the following conditions applies:
</p>
<ul>
<li>
<code>x</code> is in the set of values <a href="#Types">determined</a> by <code>T</code>.
</li>
<li>
<code>T</code> is a floating-point type and <code>x</code> can be rounded to <code>T</code>'s
precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE
negative zero further simplified to an unsigned zero. Note that constant values never result
in an IEEE negative zero, NaN, or infinity.
</li>
<li>
<code>T</code> is a complex type, and <code>x</code>'s
<a href="#Complex_numbers">components</a> <code>real(x)</code> and <code>imag(x)</code>
are representable by values of <code>T</code>'s component type (<code>float32</code> or
<code>float64</code>).
</li>
</ul>
<pre>
x T x is representable by a value of T because
'a' byte 97 is in the set of byte values
97 rune rune is an alias for int32, and 97 is in the set of 32-bit integers
"foo" string "foo" is in the set of string values
1024 int16 1024 is in the set of 16-bit integers
42.0 byte 42 is in the set of unsigned 8-bit integers
1e10 uint64 10000000000 is in the set of unsigned 64-bit integers
2.718281828459045 float32 2.718281828459045 rounds to 2.7182817 which is in the set of float32 values
-1e-1000 float64 -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0
0i int 0 is an integer value
(42 + 0i) float32 42.0 (with zero imaginary part) is in the set of float32 values
</pre>
<pre>
x T x is not representable by a value of T because
0 bool 0 is not in the set of boolean values
'a' string 'a' is a rune, it is not in the set of string values
1024 byte 1024 is not in the set of unsigned 8-bit integers
-1 uint16 -1 is not in the set of unsigned 16-bit integers
1.1 int 1.1 is not an integer value
42i float32 (0 + 42i) is not in the set of float32 values
1e1000 float64 1e1000 overflows to IEEE +Inf after rounding
</pre>
<h2 id="Blocks">Blocks</h2>
<p>
......@@ -2348,7 +2401,8 @@ For array and slice literals the following rules apply:
its position in the array.
</li>
<li>An element with a key uses the key as its index. The
key must be a non-negative constant representable by
key must be a non-negative constant
<a href="#Representability">representable</a> by
a value of type <code>int</code>; and if it is typed
it must be of integer type.
</li>
......@@ -2925,7 +2979,8 @@ If <code>a</code> is not a map:
it is <i>in range</i> if <code>0 &lt;= x &lt; len(a)</code>,
otherwise it is <i>out of range</i></li>
<li>a <a href="#Constants">constant</a> index must be non-negative
and representable by a value of type <code>int</code>
and <a href="#Representability">representable</a> by a value
of type <code>int</code></li>
</ul>
<p>
......@@ -3075,7 +3130,8 @@ For arrays or strings, the indices are <i>in range</i> if
<code>0</code> &lt;= <code>low</code> &lt;= <code>high</code> &lt;= <code>len(a)</code>,
otherwise they are <i>out of range</i>.
For slices, the upper index bound is the slice capacity <code>cap(a)</code> rather than the length.
A <a href="#Constants">constant</a> index must be non-negative and representable by a value of type
A <a href="#Constants">constant</a> index must be non-negative and
<a href="#Representability">representable</a> by a value of type
<code>int</code>; for arrays or constant strings, constant indices must also be in range.
If both indices are constant, they must satisfy <code>low &lt;= high</code>.
If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
......@@ -3135,7 +3191,8 @@ If the sliced operand is an array, it must be <a href="#Address_operators">addre
<p>
The indices are <i>in range</i> if <code>0 &lt;= low &lt;= high &lt;= max &lt;= cap(a)</code>,
otherwise they are <i>out of range</i>.
A <a href="#Constants">constant</a> index must be non-negative and representable by a value of type
A <a href="#Constants">constant</a> index must be non-negative and
<a href="#Representability">representable</a> by a value of type
<code>int</code>; for arrays, constant indices must also be in range.
If multiple indices are constant, the constants that are present must be in range relative to each
other.
......@@ -3384,7 +3441,8 @@ to the type of the other operand.
<p>
The right operand in a shift expression must have unsigned integer type
or be an untyped constant representable by a value of type <code>uint</code>.
or be an untyped constant <a href="#Representability">representable</a> by a
value of type <code>uint</code>.
If the left operand of a non-constant shift expression is an untyped constant,
it is first converted to the type it would assume if the shift expression were
replaced by its left operand alone.
......@@ -3877,30 +3935,14 @@ func() int(x) // x is converted to func() int (unambiguous)
<p>
A <a href="#Constants">constant</a> value <code>x</code> can be converted to
type <code>T</code> in any of these cases:
type <code>T</code> if <code>x</code> is <a href="#Representability">representable</a>
by a value of <code>T</code>.
As a special case, an integer constant <code>x</code> can be converted to a
<a href="#String_types">string type</a> using the
<a href="#Conversions_to_and_from_a_string_type">same rule</a>
as for non-constant <code>x</code>.
</p>
<ul>
<li>
<code>x</code> is representable by a value of type <code>T</code>.
</li>
<li>
<code>x</code> is a floating-point constant,
<code>T</code> is a floating-point type,
and <code>x</code> is representable by a value
of type <code>T</code> after rounding using
IEEE 754 round-to-even rules, but with an IEEE <code>-0.0</code>
further rounded to an unsigned <code>0.0</code>.
The constant <code>T(x)</code> is the rounded value.
</li>
<li>
<code>x</code> is an integer constant and <code>T</code> is a
<a href="#String_types">string type</a>.
The <a href="#Conversions_to_and_from_a_string_type">same rule</a>
as for non-constant <code>x</code> applies in this case.
</li>
</ul>
<p>
Converting a constant yields a typed constant as result.
</p>
......@@ -4187,7 +4229,8 @@ The divisor of a constant division or remainder operation must not be zero:
</pre>
<p>
The values of <i>typed</i> constants must always be accurately representable as values
The values of <i>typed</i> constants must always be accurately
<a href="#Representability">representable</a> by values
of the constant type. The following constant expressions are illegal:
</p>
......@@ -5683,7 +5726,7 @@ make(T, n) channel buffered channel of type T, buffer size n
<p>
The size arguments <code>n</code> and <code>m</code> must be of integer type or untyped.
A <a href="#Constants">constant</a> size argument must be non-negative and
representable by a value of type <code>int</code>.
<a href="#Representability">representable</a> by a value of type <code>int</code>.
If both <code>n</code> and <code>m</code> are provided and are constant, then
<code>n</code> must be no larger than <code>m</code>.
If <code>n</code> is negative or larger than <code>m</code> at run time,
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment