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HoldRest

Status: Experimental

present and registered, but lightly documented and not yet covered by dedicated tests.

Description

HoldRest

is an attribute that specifies that all but the first argument to a function are to be maintained in an unevaluated form.

Notes Use Evaluate to evaluate a held argument in a controlled way.

Examples (3)

Every input below was run against the current Mathilda build and its output recorded.

Applications (3)

Set the attribute on a head

In[1]:= ClearAll[g]; SetAttributes[g, HoldRest]; Attributes[g]
Out[1]= {HoldRest}

First arg evaluates, the rest are held

In[2]:= ClearAll[g]; SetAttributes[g, HoldRest]; g[1 + 1, 2 + 2]
Out[2]= g[2, 2 + 2]

If holds its branches

In[3]:= MemberQ[Attributes[If], HoldRest]
Out[3]= True

Implementation notes

Definition. HoldRest is an attribute symbol. Set on a function head, it tells the evaluator to keep every argument except the first unevaluated. The first argument is evaluated as usual. It has no builtin ā€” it is a name recognised by the attribute system and surfaced through Attributes / SetAttributes.

Representation. A bare EXPR_SYMBOL that maps to the bitflag ATTR_HOLDREST in src/attr.c. During evaluate_step, after the head's attributes are read, the argument-evaluation loop skips the held positions: with HoldRest only argument 1 is evaluated and arguments 2..n are passed through literally. Hold-family flags compose with the others (Listable, Flat, ...) the generic evaluator consults.

Usage & limits. It is the complement of HoldFirst and the partial form of HoldAll. Built-ins that carry it include control constructs such as If (the branches must not evaluate until the condition selects one) and RuleDelayed. Use Evaluate to force a held argument to evaluate in a controlled way. Like all attributes it is global to the head and is cleared by ClearAll or ClearAttributes.

Attributes: HoldRest, Protected.

References

Notes & additional examples

Notes

HoldRest is an attribute: when a head has it, every argument except the first is kept unevaluated. In g[1 + 1, 2 + 2] above, 1 + 1 evaluates to 2 while 2 + 2 is left as written, giving g[2, 2 + 2].

It is the complement of HoldFirst and the partial form of HoldAll. Control constructs such as If carry it so a branch does not evaluate until the condition picks it; RuleDelayed carries it too. Use Evaluate to force a held argument to evaluate in a controlled way. Like all attributes it is global to the head and is cleared by ClearAll or ClearAttributes.