HoldAll

HoldAll

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

Details

  • 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.

Examples

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Basic Examples  (1)

Applications  (3)

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:

Properties & Relations  (13)

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:

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 functions with the attribute HoldAll:

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:

Wolfram Research (1988), HoldAll, Wolfram Language function, https://reference.wolfram.com/language/ref/HoldAll.html.

Text

Wolfram Research (1988), HoldAll, Wolfram Language function, https://reference.wolfram.com/language/ref/HoldAll.html.

CMS

Wolfram Language. 1988. "HoldAll." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/HoldAll.html.

APA

Wolfram Language. (1988). HoldAll. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/HoldAll.html

BibTeX

@misc{reference.wolfram_2023_holdall, author="Wolfram Research", title="{HoldAll}", year="1988", howpublished="\url{https://reference.wolfram.com/language/ref/HoldAll.html}", note=[Accessed: 18-March-2024 ]}

BibLaTeX

@online{reference.wolfram_2023_holdall, organization={Wolfram Research}, title={HoldAll}, year={1988}, url={https://reference.wolfram.com/language/ref/HoldAll.html}, note=[Accessed: 18-March-2024 ]}