Evaluation strategies are used by programming languages to determine two things—when to evaluate the arguments of a function call and what kind of value to pass to the function.
To illustrate, a function application may evaluate the argument before evaluating the function's body and pass the ability to look up the argument's current value and modify it via assignment.
In practical terms, many modern programming languages like C# and Java have converged on a call-by-value/call-by-reference evaluation strategy for function calls.
Historically, call by value and call by name date back to ALGOL 60, which was designed in the late 1950s.
Evaluation strategy is specified by the programming language definition, and is not a function of any specific implementation.
Main article: Eager evaluation
In strict evaluation, the arguments to a function are always evaluated completely before the function is applied.
Most existing programming languages use strict evaluation for functions.
Applicative order evaluation is an evaluation strategy in which an expression is evaluated by repeatedly evaluating its leftmost innermost reducible expression.
This means that a function's arguments are evaluated before the function is applied.
Call by value
In call by value, the argument expression is evaluated, and the resulting value is bound to the corresponding variable in the function (frequently by copying the value into a new memory region).
If the function or procedure is able to assign values to its parameters, only its local variable is assigned—that is, anything passed into a function call is unchanged in the caller's scope when the function returns.
Call by value is not a single evaluation strategy, but rather the family of evaluation strategies in which a function's argument is evaluated before being passed to the function.
While many programming languages (such as Common Lisp, Eiffel and Java) that use call by value evaluate function arguments left-to-right, some evaluate functions and their arguments right-to-left, and others (such as Scheme, OCaml and C) do not specify order.
In some cases, the term "call by value" is problematic, as the value which is passed is not the value of the variable as understood by the ordinary meaning of value, but an implementation-specific reference to the value.
The effect is that what syntactically looks like call by value may end up rather behaving like call by reference or call by sharing, often depending on very subtle aspects of the language semantics.
The reason for passing a reference is often that the language technically does not provide a value representation of complicated data, but instead represents them as a data structure while preserving some semblance of value appearance in the source code.
Exactly where the boundary is drawn between proper values and data structures masquerading as such is often hard to predict.
In C, an array (of which strings are special cases) is a data structure but the name of an array is treated as (has as value) the reference to the first element of the array, while a struct variable's name refers to a value even if it has fields that are vectors.
In Maple, a vector is a special case of a table and therefore a data structure, but a list (which gets rendered and can be indexed in exactly the same way) is a value.
In Tcl, values are "dual-ported" such that the value representation is used at the script level, and the language itself manages the corresponding data structure, if one is required.
Modifications made via the data structure are reflected back to the value representation and vice versa.
The description "call by value where the value is a reference" is common (but should not be understood as being call by reference); another term is call by sharing.
Thus the behaviour of call by value Java or Visual Basic and call by value C or Pascal are significantly different: in C or Pascal, calling a function with a large structure as an argument will cause the entire structure to be copied (except if it's actually a reference to a structure), potentially causing serious performance degradation, and mutations to the structure are invisible to the caller.
However, in Java or Visual Basic only the reference to the structure is copied, which is fast, and mutations to the structure are visible to the caller.
Call by reference
Call by reference (or pass by reference) is an evaluation strategy where a function receives an implicit reference to a variable used as argument, rather than a copy of its value.
This typically means that the function can modify (i.e., assign to) the variable used as argument—something that will be seen by its caller.
Call by reference can therefore be used to provide an additional channel of communication between the called function and the calling function.
A call-by-reference language makes it more difficult for a programmer to track the effects of a function call, and may introduce subtle bugs.
A simple litmus test for whether a language supports call-by-reference semantics is if it's possible to write a traditional swap(a, b) function in the language.
Many languages support call by reference in some form, but few use it by default.
FORTRAN II is an early example of a call-by-reference language.
C++ additionally offers call by reference to const.
Call by reference can be simulated in languages that use call by value and don't exactly support call by reference, by making use of references (objects that refer to other objects), such as pointers (objects representing the memory addresses of other objects).
It is not a separate evaluation strategy—the language calls by value—but sometimes it is referred to as "call by address" or "pass by address".
In purely functional languages there is typically no semantic difference between the two strategies (since their data structures are immutable, so there is no possibility for a function to modify any of its arguments), so they are typically described as call by value even though implementations frequently use call by reference internally for the efficiency benefits.
Following is an example that demonstrates call by reference in the E programming language:
Following is an example of call by address that simulates call by reference in C:
Call by sharing
However, the term "call by sharing" is not in common use; the terminology is inconsistent across different sources.
For example, in the Java community, they say that Java is call by value.
Because they are boxed they can be said to pass by copy of reference (where primitives are boxed before passing and unboxed at called function).
The semantics of call by sharing differ from call by reference: "In particular it is not call by value because mutations of arguments performed by the called routine will be visible to the caller.
And it is not call by reference because access is not given to the variables of the caller, but merely to certain objects".
So, for example, if a variable was passed, it is not possible to simulate an assignment on that variable in the callee's scope.
However, since the function has access to the same object as the caller (no copy is made), mutations to those objects, if the objects are mutable, within the function are visible to the caller, which may appear to differ from call by value semantics.
Mutations of a mutable object within the function are visible to the caller because the object is not copied or cloned—it is shared.
For example, in Python, lists are mutable, so:
outputs because the append method modifies the object on which it is called.
Assignments within a function are not noticeable to the caller, because, in these languages, passing the variable only means passing (access to) the actual object referred to by the variable, not access to the original (caller's) variable.
Since the rebound variable only exists within the scope of the function, the counterpart in the caller retains its original binding.
Compare the Python mutation above with the code below, which binds the formal argument to a new object:
outputs , because the statement list = reassigns a new list to the variable rather than to the location it references.
For immutable objects, there is no real difference between call by sharing and call by value, except if object identity is visible in the language.
The use of call by sharing with mutable objects is an alternative to input/output parameters: the parameter is not assigned to (the argument is not overwritten and object identity is not changed), but the object (argument) is mutated.
Although this term has widespread usage in the Python community, identical semantics in other languages such as Java and Visual Basic are often described as call by value, where the value is implied to be a reference to the object.
Call by copy-restore
Call by copy-restore—also known as "copy-in copy-out", "call by value result", "call by value return" (as termed in the Fortran community)—is a special case of call by reference where the provided reference is unique to the caller.
This variant has gained attention in multiprocessing contexts and Remote procedure call: if a parameter to a function call is a reference that might be accessible by another thread of execution, its contents may be copied to a new reference that is not; when the function call returns, the updated contents of this new reference are copied back to the original reference ("restored").
The semantics of call by copy-restore also differ from those of call by reference, where two or more function arguments alias one another (i.e., point to the same variable in the caller's environment).
Under call by reference, writing to one will affect the other; call by copy-restore avoids this by giving the function distinct copies, but leaves the result in the caller's environment undefined depending on which of the aliased arguments is copied back first—will the copies be made in left-to-right order both on entry and on return?
When the reference is passed to the callee uninitialized, this evaluation strategy may be called "call by result".
Main article: Partial evaluation
In partial evaluation, evaluation may continue into the body of a function that has not been applied.
Any sub-expressions that do not contain unbound variables are evaluated, and function applications whose argument values are known may be reduced.
If there are side effects, complete partial evaluation may produce unintended results, which is why systems that support partial evaluation tend to do so only for "pure" expressions (i.e., those without side effects) within functions.
Under "full β-reduction", any function application may be reduced (substituting the function's argument into the function using capture-avoiding substitution) at any time.
This may be done even within the body of an unapplied function.
Call by future
See also: Futures and promises
"Call by future", also known as "parallel call by name", is a concurrent evaluation strategy in which the value of a future expression is computed concurrently with the flow of the rest of the program with promises, also known as futures.
When the promise's value is needed, the main program blocks until the promise has a value (the promise or one of the promises finishes computing, if it has not already completed by then).
This strategy is non-deterministic, as the evaluation can occur at any time between creation of the future (i.e., when the expression is given) and use of the future's value.
It is similar to call by need in that the value is only computed once, and computation may be deferred until the value is needed, but it may be started before.
Further, if the value of a future is not needed, such as if it is a local variable in a function that returns, the computation may be terminated partway through.
If implemented with processes or threads, creating a future will spawn one or more new processes or threads (for the promises), accessing the value will synchronize these with the main thread, and terminating the computation of the future corresponds to killing the promises computing its value.
If implemented with a coroutine, as in .NET async/await, creating a future calls a coroutine (an async function), which may yield to the caller, and in turn be yielded back to when the value is used, cooperatively multitasking.
Optimistic evaluation is another call-by-need variant where the function's argument is partially evaluated for some amount of time (which may be adjusted at runtime).
After that time has passed, evaluation is aborted and the function is applied using call by need.
This approach avoids some the call-by-need strategy's runtime expenses while retaining desired termination characteristics.
- Beta normal form
- Comparison of programming languages
- Lambda calculus
- Parameter (computer science)
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Evaluation strategy.