A ParameterPassing mode where the actual parameter is ''copied'' into the formal parameter at the point of function invocation (for example, implemented by copying into an agreed-upon register or location on the program stack).

Has the following attributes and consequences:

* Does not introduce aliasing; caller and callee get separate copies.  
* Does not allow callee to influence state of caller (modifications to callee's formal parameter have no effect on caller; though there is a similar mode called CallByValueResult where the callee's formal parameter is passed ''back'' when the function returns).
* Can be expensive to implement for large mutable objects, especially those which cannot fit in registers.
* Forces expression evaluation at each passing point, i.e. all SideEffect''''''s are pre-call.

CallByValue refers to ParameterPassing modes with the above ''semantics'', even if implemented differently under the hood.

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See ParameterPassing, CallByReference, ExplicitLazyProgramming, CallByThunk
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CategoryLanguageFeature