Name Resolution Rules
- An expression consisting of two relations connected by and then or or
else (a short-circuit control form) shall resolve to be of some boolean type;
the expected type for both relations is that same boolean type.
- The following logical operators are predefined for every boolean type T,
for every modular type T, and for every one-dimensional array type T whose
component type is a boolean type:
function "and"(Left, Right : T) return T
function "or" (Left, Right : T) return T
function "xor"(Left, Right : T) return T
- For boolean types, the predefined logical operators and, or, and xor
perform the conventional operations of conjunction, inclusive disjunction,
and exclusive disjunction, respectively.
- For modular types, the predefined logical operators are defined on a
bit-by-bit basis, using the binary representation of the value of the
operands to yield a binary representation for the result, where zero
represents False and one represents True. If this result is outside the base
range of the type, a final subtraction by the modulus is performed to bring
the result into the base range of the type.
- The logical operators on arrays are performed on a component-by-component
basis on matching components (as for equality -- see
4.5.2), using the predefined logical operator for the component type.
The bounds of the resulting array are those of the left operand.
- The short-circuit control forms and then and or else deliver the same
result as the corresponding predefined and and or operators for boolean
types, except that the left operand is always evaluated first, and the right
operand is not evaluated if the value of the left operand determines the
- For the logical operators on arrays, a check is made that for each
component of the left operand there is a matching component of the right
operand, and vice versa. Also, a check is made that each component of the
result belongs to the component subtype. The exception Constraint_Error is
raised if either of the above checks fails.
(12) The conventional meaning of the logical operators is given by the
following truth table:
A B (A and B) (A or B) (A xor B)
True True True True False
True False False True True
False True False True True
False False False False False
- Examples of logical operators:
Sunny or Warm
Filter(1 .. 10) and Filter(15 .. 24) -- see 3.6.1
- Examples of short-circuit control forms:
Next_Car.Owner /= null and then Next_Car.Owner.Age > 25 -- see 3.10.1
N = 0 or else A(N) = Hit_Value
-- Email comments, additions, corrections, gripes, kudos, etc. to:
Magnus Kempe -- Magnus.Kempe@di.epfl.ch
Page last generated: 95-03-12