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Concept
Evaluation strategy
Summary
In a programming language, an evaluation strategy is a set of rules for evaluating expressions. The term is often used to refer to the more specific notion of a parameter-passing strategy that defines the kind of value that is passed to the function for each parameter (the binding strategy) and whether to evaluate the parameters of a function call, and if so in what order (the evaluation order). The notion of reduction strategy is distinct, although some authors conflate the two terms and the definition of each term is not widely agreed upon.
To illustrate, executing a function call f(a,b) may first evaluate the arguments a and b, store the results in references or memory locations ref_a and ref_b, then evaluate the function's body with those references passed in. This gives the function the ability to look up the argument values, to modify them via assignment as if they were local variables, and to return values via the references. This is the call-by-reference evaluation strategy.
Evaluation strategy is part of the semantics of the programming language definition. Some languages, such as PureScript, have variants with different evaluation strategies. Some declarative languages, such as Datalog, support multiple evaluation strategies. Some languages define a calling convention.
This is a table of evaluation strategies and representative languages by year introduced. The representative languages are listed in chronological order, starting with the language(s) that introduced the strategy and followed by prominent languages that use the strategy.
While the order of operations defines the abstract syntax tree of the expression, the evaluation order defines the order in which expressions are evaluated. For example, the Python program
def f(x):
print(x)
return x
f(1) + f(2)
outputs 1 2 due to Python's left-to-right evaluation order, but a similar program in OCaml:
let f x = print_string (string_of_int x); x ;;
f 1 + f 2
outputs 2 1 due to OCaml's right-to-left evaluation order.
Official source
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