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.