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clarify conversions vs type guards.
allow conversions between equal types. R=r DELTA=15 (4 added, 4 deleted, 7 changed) OCL=25618 CL=25630
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@ -3609,32 +3609,36 @@ T(value)
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</pre>
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<p>
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where <code>T</code> is the type name of an arithmetic type or string (§Basic types),
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and <code>value</code> is the value of an expression that can be converted to a value
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where <code>T</code> is a type
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and <code>value</code> is an expression
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that can be converted to a value
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of result type <code>T</code>.
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<p>
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The following conversion rules apply:
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</p>
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<ul>
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<li>
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1) Between integer types. If the value is a signed quantity, it is
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1) Between equal types. The conversion always succeeds.
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</li>
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<li>
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2) Between integer types. If the value is a signed quantity, it is
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sign extended to implicit infinite precision; otherwise it is zero
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extended. It is then truncated to fit in the result type size.
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For example, <code>uint32(int8(0xFF))</code> is <code>0xFFFFFFFF</code>.
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The conversion always yields a valid value; there is no signal for overflow.
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</li>
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<li>
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2) Between integer and floating point types, or between floating point
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3) Between integer and floating point types, or between floating point
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types. To avoid overdefining the properties of the conversion, for
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now it is defined as a ``best effort'' conversion. The conversion
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always succeeds but the value may be a NaN or other problematic
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result. <font color=red>TODO: clarify?</font>
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</li>
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<li>
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3) Strings permit two special conversions.
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4) Strings permit two special conversions.
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</li>
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<li>
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3a) Converting an integer value yields a string containing the UTF-8
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4a) Converting an integer value yields a string containing the UTF-8
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representation of the integer.
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(TODO: this one could be done just as well by a library.)
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@ -3644,7 +3648,7 @@ string(0x65e5) // "\u65e5"
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</li>
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<li>
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3b) Converting an array or slice of bytes yields a string whose successive
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4b) Converting an array or slice of bytes yields a string whose successive
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bytes are those of the array/slice.
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<pre>
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@ -3658,10 +3662,6 @@ There is no linguistic mechanism to convert between pointers and integers.
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The <code>unsafe</code> package
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implements this functionality under
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restricted circumstances (§Package <code>unsafe</code>).
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<font color=red>
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TODO: Do we allow interface/ptr conversions in this form or do they
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have to be written as type guards? (§Type guards)
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</font>
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</p>
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