BUILT-IN MATHEMATICA SYMBOL

HoldAll

HoldAll
is an attribute which specifies that all arguments to a function are to be maintained in an unevaluated form.

You can use Evaluate to evaluate the arguments of a HoldAll function in a controlled way.

Even when a function has attribute HoldAll, Sequence objects that appear in its arguments are still by default flattened, Unevaluated wrappers are stripped, and upvalues associated with the arguments are used.

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Many functions with scoping behavior have the HoldAll attribute:

Plotting lists of functions will use separate styling for the different functions:

If the list structure is not manifest, no separate styling is provided:

Use Evaluate to make the list structure manifest:

Different vector-valued functions in a list will still get separate styling:

Use HoldAll and Unevaluated to suppress evaluation of symbols wherever it would occur:

Find the length of a symbol's name even if it has a value:

Implement your own control structure:

Hold is a container with the attribute HoldAll:

Functions that operate on symbols often need the HoldAll attribute:

Without the attribute, they would operate on the symbol's value:

Control structures such as Table protect their arguments from evaluation:

Otherwise, global values might interfere with their operation:

Use Evaluate to force evaluation of an argument of a HoldAll function:

Force evaluation of the right-hand side of a delayed definition:

Use Unevaluated to temporarily treat a function as if it had the attribute HoldAll:

Unevaluated inside a held expression is not removed:

This is the full form of the evaluation result:

This is the full form of the input, before evaluation:

Suppress the evaluation of the arguments of a pure function:

Sequence splicing still happens for HoldAll functions:

Use the container HoldComplete to suppress even such transformations:

The attribute that causes this behavior is HoldAllComplete:

Substitution works inside a held expression:

Insert into a held expression:

NHoldAll protects arguments from N, but evaluates them normally otherwise:

HoldPattern protects patterns from evaluation, but does not interfere with pattern matching: