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Protect

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description

Protect[s1, s2, ...]

sets the attribute Protected for the named symbols and returns the list of their names. Protect[{s1, s2, ...}] accepts a list of specs.

Notes Protect has attribute HoldAll.

Examples (12)

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

Basic examples (5)

In[1]:= f[x_] := x^2; Protect[f]
Out[1]= {"f"}

In[2]:= f[x_, y_] := x + y

In[3]:= Unprotect[f]
Out[3]= {"f"}

In[4]:= Protect[f]; Protect[f]
Out[4]= {}

In[5]:= Check[f[x_] := 0, "refused"]
Out[5]= "refused"

Applications (7)

Returns the names whose Protected attribute was newly set

In[6]:= Protect[foo, bar]
Out[6]= {"foo", "bar"}

In[7]:= MemberQ[Attributes[foo], Protected]
Out[7]= True

Already protected: nothing changed

In[8]:= Protect[foo]
Out[8]= {}

Assignment to a protected symbol emits Set::wrsym

In[9]:= Check[foo = 6, "refused"]
Out[9]= "refused"

The value was not assigned

In[10]:= foo
Out[10]= foo

A list of specs; strings name symbols

In[11]:= Protect[{"k1", k2}]
Out[11]= {"k1", "k2"}

In[12]:= Attributes[Protect]
Out[12]= {HoldAll, Protected}

Implementation notes

Algorithm. builtin_protect (src/core.c) calls the shared driver core_protect_unprotect(res, protecting = true). That driver walks the argument list ā€” each spec being a symbol, a string, or a flat List of them ā€” and applies core_protect_one to every name. core_protect_one does nothing if the symbol is already Protected; otherwise it sets the ATTR_PROTECTED bit and bumps the rule epoch (invalidating the evaluation cache), returning true only when the bit was newly set.

The driver collects the names whose state actually changed into a growable Expr** buffer and returns them as a List of strings ā€” matching the Wolfram convention where Protect reports exactly what it altered, so re-protecting an already-protected symbol yields {}. The Protected attribute is what the evaluator and Set consult to refuse redefinition of a symbol.

Attributes & limits. Protect carries HoldAll | Protected, so its arguments reach the handler as unevaluated symbol names. Assigning to a protected symbol emits Set::wrsym / SetDelayed::wrsym through the message funnel, so Check sees the refusal. Protected is the only sealing attribute (since v0.301), and Unprotect always lifts it.

  • Both have attributes {HoldAll, Protected} and hold their arguments.
  • The typical sequence for adding rules to an existing symbol is Unprotect[f]; definition; Protect[f].

Attributes: HoldAll, Protected.

References

See also: Unprotect

Notes & additional examples

Notes

  • Protect[s1, s2, ...] sets the Protected attribute on each symbol. That is what Set, SetDelayed, Unset, ClearAll, Remove and SetAttributes consult before changing a symbol.
  • It returns the names, as strings, whose state actually changed, so protecting an already-protected symbol gives {}.
  • Arguments may be symbols, strings, or a flat list of them. HoldAll | Protected.
  • Every built-in carries Protected, and Unprotect always lifts it. No attribute seals a symbol permanently (since v0.301).
  • See also: Unprotect, Attributes, SetAttributes.