Evaluation

The Standard Evaluation SequencePreventing Evaluation
Nonstandard Argument EvaluationGlobal Control of Evaluation
Overriding Nonstandard Argument EvaluationAborts
The Standard Evaluation Sequence
The following is the sequence of steps that the Wolfram Language follows in evaluating an expression like h[e1,e2…]. Every time the expression changes, the Wolfram Language effectively starts the evaluation sequence over again.
Nonstandard Argument Evaluation
There are a number of built‐in Wolfram Language functions that evaluate their arguments in special ways. The control structure While is an example. The symbol While has the attribute HoldAll. As a result, the arguments of While are not evaluated as part of the standard evaluation process. Instead, the internal code for While evaluates the arguments in a special way. In the case of While, the code evaluates the arguments repeatedly, so as to implement a loop.
Control structures
arguments evaluated in a sequence determined by control flow (e.g. CompoundExpression )
Conditionals
arguments evaluated only when they correspond to branches that are taken (e.g. If , Which )
Logical operations
arguments evaluated only when they are needed in determining the logical result (e.g. And , Or )
Iteration functions
first argument evaluated for each step in the iteration (e.g. Do , Sum , Plot )
Tracing functions
form never evaluated (e.g. Trace )
Assignments
first argument only partially evaluated (e.g. Set , AddTo )
Pure functions
function body not evaluated (e.g. Function )
Scoping constructs
variable specifications not evaluated (e.g. Module , Block )
Holding functions
argument maintained in unevaluated form (e.g. Hold , HoldPattern )
Built‐in functions that evaluate their arguments in special ways.

Logical Operations

In an expression of the form the are evaluated in order. As soon as any is found to be False, evaluation is stopped, and the result False is returned. This means that you can use the to represent different “branches” in a program, with a particular branch being evaluated only if certain conditions are met.
The Or function works much like And; it returns True as soon as it finds any argument that is True. Xor, on the other hand, always evaluates all its arguments.

Iteration Functions

An iteration function such as Do[f,{i,imin,imax}] is evaluated as follows:
If there are several iteration variables, the same procedure is followed for each variable in turn, for every value of all the preceding variables.
Unless otherwise specified, f is not evaluated until a specific value has been assigned to i, and is then evaluated for each value of i chosen. You can use Evaluate[f] to make f be evaluated immediately, rather than only after a specific value has been assigned to i.

Assignments

The left‐hand sides of assignments are only partially evaluated.
The right‐hand side is evaluated for immediate (=), but not for delayed (:=), assignments.
Any subexpression of the form HoldPattern[expr] that appears on the left‐hand side of an assignment is not evaluated. When the subexpression is used for pattern matching, it matches as though it were expr without the HoldPattern.
Overriding Nonstandard Argument Evaluation
f[…,Evaluate[expr],…]
evaluates the argument expr, whether or not f has a HoldFirst, HoldRest, or HoldAll attribute specifying that it should be held
Overriding holding of arguments.
By using Evaluate, you can get any argument of a function evaluated immediately, even if the argument would usually be evaluated later under the control of the function. An exception to this is when the function has the HoldAllComplete attribute; in this case, the contents of the function are not modified by the evaluator.
Preventing Evaluation
The Wolfram Language provides various functions which act as "wrappers" to prevent the expressions they contain from being evaluated.
Hold[expr]
treated as Hold[expr] in all cases
HoldComplete[expr]
treated as HoldComplete[expr] with upvalues disabled
HoldForm[expr]
treated as expr for printing
HoldPattern[expr]
treated as expr in rules, definitions and patterns
Unevaluated[expr]
treated as expr when arguments are passed to a function
Wrappers that prevent expressions from being evaluated.
Global Control of Evaluation
In the evaluation procedure described so far, two basic kinds of steps are involved:
Iteration leads to evaluation chains in which successive expressions are obtained by the application of various transformation rules.
Trace shows evaluation chains as lists, and shows subsidiary evaluations corresponding to recursion in sublists.
The expressions associated with the sequence of subsidiary evaluations which lead to an expression currently being evaluated are given in the list returned by Stack[].
$RecursionLimit
maximum recursion depth
$IterationLimit
maximum number of iterations
Global variables controlling the evaluation of expressions.
Aborts
You can ask the Wolfram Language to abort at any point in a computation, either by calling the function Abort[], or by typing appropriate interrupt keys.
When asked to abort, the Wolfram Language will terminate the computation as quickly as possible. If the answer obtained would be incorrect or incomplete, then the Wolfram Language returns $Aborted instead of giving that answer.
Aborts can be caught using CheckAbort, and can be postponed using AbortProtect.