'''`Tcl_ConvertToType`''', a function in the [Tcl C API], converts a value from
one type to another, if possible.


[KBK] said the following in the [Tcl Chatroom], 2014-06-19:


The problem with Tcl_ConvertToType isn't performance. It's future-proofing.
There was a lot of code out there that assumed it could Tcl_ConvertToType to
force the object {12345678901234567890} to a double, leaving a 'double'
internal rep. In the 8.4/8.5 reworkings of numeric handling, we introduced
first 64-bit ints and then arbitrary-precision ints... and suddenly, it was
much more convenient to leave numbers in a 'canonical' form and have
Tcl_GetFooFromObj do the casting.  Otherwise, we had repeated string
conversions, because we couldn't just cast the double {123456678901234567890}
back to an integer; it lost precision.  So we adopted the rule that the
'internal rep' belongs to whatever code creates it; others are expected to
Tcl_GetWhateverFromObj to extract it.  And, if it can happen to something as
simple as integers, it can happen to anything.