BooleanQ¶

Status: Experimental

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

Description¶

BooleanQ[expr]

gives True if expr is either True or False, and False otherwise. Unlike TrueQ it tests the symbol, so BooleanQ[False] is True.

Examples (4)¶

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

Applications (4)¶

In[1]:= BooleanQ[True]
Out[1]= True

The inequality reduces to True first

In[2]:= BooleanQ[2 > 1]
Out[2]= True

A plain symbol is not a boolean

In[3]:= BooleanQ[x]
Out[3]= False

Nor is a number

In[4]:= BooleanQ[1]
Out[4]= False

Implementation notes¶

Algorithm. builtin_booleanq is a one-argument predicate. It checks that its single argument is an EXPR_SYMBOL whose interned name is either SYM_True or SYM_False, and returns the symbol True or False accordingly. The argument is evaluated first (ordinary, non-held evaluation), so BooleanQ[2 > 1] sees the already reduced True. A wrong argument count routes through builtin_arg_error.

Data structures. Plain pointer comparison against the two interned boolean names; no allocation beyond the returned result symbol.

Complexity / limits. O(1). Attributes: Protected. Unlike TrueQ — which asks "does this reduce to True?" and so maps everything that is not True to False — BooleanQ tests the symbol itself, so BooleanQ[False] is True while TrueQ[False] is False. Any non-boolean (a number, an unevaluated symbol, an unreduced inequality) gives False.

Attributes: Protected.

References¶

Source: src/core.c
Specification index: Mathilda_spec.md

Notes & additional examples¶

Notes¶

BooleanQ[expr] gives True when expr is literally the symbol True or False, and False for everything else. The argument is evaluated first, so BooleanQ[2 > 1] sees the reduced True.

It tests the symbol, which is the key difference from TrueQ: BooleanQ[False] is True (it is a boolean), whereas TrueQ[False] is False (it is not True). BooleanQ is Protected.