Next: 3.5 Magnitudes and Numbers
Up: 3 The Slate World
Previous: 3.3 Traits
  Contents
Subsections
Slate's interpreter primitively provides the objects True
and False, which are clones of Boolean, and delegate
to Boolean traits. Logical methods are defined on these
in a very minimalistic way. Table cap:Basic-Logical-Operators
shows the non-lazy logical methods and their meanings.
Table:
Basic Logical Operators
Description |
Selector |
AND/Conjunction |
/\ |
OR/Disjunction |
\/ |
NOT/Negation |
not |
EQV/Equivalence |
eqv: |
XOR/Exclusive-OR |
xor: |
|
Logical methods are provided which take a block as their second argument
(and:, or:, xor:, eqv:). By accepting
a block as the second argument, they can and do provide conditional
evaluation of the second argument only in the case that the first
does not decide the total result automatically4. Blocks that evaluate logical expressions can be used lazily in these
logical expressions. For example,
-
- (x < 3) and: [y > 7].
only evaluates the right-hand block argument if the first argument
turns out to be True.
-
- (x < 3) or: [y > 7].
only evaluates the right-hand block argument if the first argument
turns out to be False.
In general, the basic of booleans to switch between code alternatives
is to use ifTrue:, ifFalse:, and ifTrue:ifFalse:
for the various combinations of binary branches. For example,
-
- x isNegative ifTrue: [x: x negated].
ensures that x is positive by optionally executing code to
make it positive if it's not. Of course if only the result is desired,
instead of just the side-effect, the entire expression's result will
be the result of the executed block, so that it can be embedded in
further expressions.
Conditional evaluation can also be driven by whether or not a slot
has been initialized, or whether a method returns Nil. There
are a few options for conditionalizing on Nil:
- expr ifNil: block
- and expr ifNotNil: block
execute their blocks based on whether the expression evaluates to
Nil, and returns the result.
- expr ifNil: nilBlock ifNotNil: otherBlock
- provides
both options in one expression.
- expr ifNotNilDo: block
- applies the block to the expression's
result if it turns out to be non-Nil, so the block given
must accept one argument. ifNil:ifNotNilDo: is also
provided for completeness.
Slate includes various idioms for constructing basic loops.
- n timesRepeat: block
- executes the block n times.
- condition whileTrue: block
- and condition whileFalse: block
execute their blocks repeatedly, checking the condition before each
iteration.
- whileTrue
- and whileFalse execute their
blocks repeatedly, checking the return value before repeating iterations.
- a upTo: b do: block
- and b downTo: a do: block
executes the block with each number in turn from a to b,
inclusive.
- upTo:by:do:
- and downTo:by:do: executes
the block with each number in turn in the inclusive range, with the
given stepping increment.
- a below: b do: block
- and b above: a do: block
act identically to the previous method except that they stop just
before the last value. This assists in iterating over array ranges,
where the 0-based indexing makes a difference in range addresses by
one, avoiding excessive use of size - 1 calls.
Slate's looping control structures can easily be extended without
concern due to the fact that the interpreter unrolls properly tail-recursive
blocks into low-level loop code that re-uses the same activation frame.
So basically structuring custom looping code so that it calls itself
last within its own body and returns that value will avoid the need
for increasing stack space per iteration.
Next: 3.5 Magnitudes and Numbers
Up: 3 The Slate World
Previous: 3.3 Traits
  Contents
The Slate Project
2004-08-06