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conversions to unsigned numbers are not checked anymore; implements /… (#12688) [backport]

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* conversions to unsigned numbers are not checked anymore; implements / fixes https://github.com/nim-lang/RFCs/issues/175

* change the spec yet again to be less consistent but to make more sense; updated the changelog
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Andreas Rumpf 2019-11-20 17:08:43 +01:00 • committed by GitHub
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doc/manual.rst
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@ -3188,6 +3188,7 @@ has lots of advantages:
Type conversions
----------------
Syntactically a `type conversion` is like a procedure call, but a
type name replaces the procedure name. A type conversion is always
safe in the sense that a failure to convert a type to another
@ -3207,6 +3208,19 @@ A type conversion can also be used to disambiguate overloaded routines:
let procVar = (proc(x: string))(p)
procVar("a")
Since operations on unsigned numbers wrap around and are unchecked so are
type conversion to unsigned integers and between unsigned integers. The
rationale for this is mostly better interoperability with the C Programming
language when algorithms are ported from C to Nim.
Exception: Values that are converted to an unsigned type at compile time
are checked so that code like ``byte(-1)`` does not compile.
**Note**: Historically the operations
were unchecked and the conversions were sometimes checked but starting with
the revision 1.0.4 of this document and the language implementation the
conversions too are now *always unchecked*.
Type casts
----------