Skip to content

HoldFirst

Status: Experimental

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

Description

HoldFirst

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

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

Examples (4)

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

Applications (4)

An attribute symbol carries itself, plus Protected

In[1]:= Attributes[HoldFirst]
Out[1]= {HoldFirst, Protected}

Give a test function the attribute

In[2]:= SetAttributes[holdfn, HoldFirst]

The first argument is held, the rest evaluate

In[3]:= holdfn[1 + 1, 2 + 2]
Out[3]= holdfn[1 + 1, 4]

And the function now reports it

In[4]:= Attributes[holdfn]
Out[4]= {HoldFirst}

Implementation notes

Definition. HoldFirst is one of Mathilda's evaluation attributes: set on a symbol f, it tells the evaluator to keep the first argument of f[...] unevaluated while evaluating the rest. It is a name the attribute machinery recognises, not a function ā€” it has no builtin and no value of its own, and it is itself Protected (its own attributes are {HoldFirst, Protected}).

Representation. Internally it is the bit ATTR_HOLDFIRST in a symbol's attribute bitmask. string_to_attribute("HoldFirst") (src/attr.c) maps the name to that bit, so SetAttributes[f, HoldFirst] and ClearAttributes flip it; Attributes[f] reads the mask back as a list of names. The default attribute table in attr.c already carries it for the built-ins that need it (Set, AppendTo, PrependTo, MessageName, Pattern, SetAttributes, ...). Changing it bumps the rule epoch so any cached evaluation of f is invalidated.

Usage & limits. When the evaluator processes f[a1, a2, ...] it reads f's attributes before evaluating arguments; with HoldFirst, a1 is left as written and a2, ... evaluate normally (step 3 of the evaluator loop). Evaluate[a1] overrides the hold locally. It is the single-argument-held member of the HoldFirst / HoldRest / HoldAll family; use HoldAll to hold every argument, HoldRest to hold all but the first.

Attributes: HoldFirst, Protected.

References

Notes & additional examples

Notes

HoldFirst is an evaluation attribute. Setting it on a symbol f makes the evaluator keep f's first argument unevaluated while evaluating the others, which is exactly what assignment heads such as Set, AppendTo and SetAttributes rely on. It is one bit (ATTR_HOLDFIRST) in a symbol's attribute mask; SetAttributes / ClearAttributes flip it and Attributes reads it back. Evaluate can override a hold locally. It is the first-argument member of the HoldFirst / HoldRest / HoldAll family.