Conditional Expressions and Predicates
The expressive power of the class of functions that we can define at this point is very limited, because we have no way to make tests and to perform different operations depending on the result of a test. For instance, we cannot define a function that computes the absolute value of a number by testing whether the number is positive, negative, or zero and taking different actions in the different cases according to the rule
This construct is called a case analysis, and there are special forms in LFE for notating such case analyses:
cond
if
case
- pattern matching and guards
We will now explore those.
The cond
Form
It is called cond
(which stands for "conditional"), and it is used as follows:
(defun abs (x)
(cond ((> x 0) x)
((== x 0) 0)
((< x 0) (- x))))
In non-LFE Lisps, the general form of a conditional expression is
(cond (<p1> <e1>)
(<p2> <e2>)
(<p3> <e3>)
...
(<pn> <en>))
consisting of the symbol cond followed by parenthesized pairs of expressions (<p> <e>)
called clauses. The first expression in each pair is a predicate -- that is, an expression whose value is interpreted as either true or false.1
Thanks to that fact that Erlang underlies LFE, the LFE cond
supports both pattern-matching and what Erlang calls "guards". As such, the predicates in LFE cond
s may take the following additional forms:
(cond (<p1> <e1>)
((?= <pattern2> <p2>) <e2>)
((?= <pattern3> <guard3> <p3>) <e3>)
...
((?= <patternn> <guardn> <pn>) <en>))
Conditional expressions are evaluated as follows. The predicate <p1>
is evaluated first. If its value is false, then<p2>
is evaluated. If <p2>
's value is also false, then <p3>
is evaluated. This process continues until a predicate is found whose value is true, in which case the interpreter returns the value of the corresponding consequent expression <e>
of the clause as the value of the conditional expression. If none of the <p>
's is found to be true
, the value of the cond
is false
.
The word predicate is used for functions that return true
or false
, as well as for expressions that evaluate to true
or false
. The absolute-value function abs
makes use of the primitive predicates >
, <
, and ==
.2 These take two numbers as arguments and test whether the first number is, respectively, greater than, less than, or equal to the second number, returning true or false accordingly.
Another way to write the absolute-value function is
(defun abs (x)
(cond ((< x 0) (- x))
('true x)))
which could be expressed in English as "If x
is less than zero return -x
; otherwise return x
." Since in LFE a cond
with no true
predicates returns false
, if we want a final, "default" value, we need to provide a predicate that always evaluates to true
. The simplest such predicate is true
.
The if
Form
Another condition form available to most Lisps and to LFE is if
. Here is yet another way to write the absolute-value function:
(defun abs (x)
(if (< x 0)
(- x)
x))
This uses the special form if
, a restricted type of conditional that can be used when there are precisely two cases in the case analysis. The general form of an if expression is
(if <predicate> <consequent> <alternative>)
To evaluate an if
expression, the interpreter starts by evaluating the <predicate>
part of the expression. If the <predicate>
evaluates to a true
value, the interpreter then evaluates the <consequent>
and returns its value. Otherwise it evaluates the <alternative>
and returns its value.3
The case
Form
Through Erlang, LFE supports a form not found by default in most Lisps: case
. case
takes an expression and then provides conditions based on matches for that expression. Here is the general form:
(case <expression>
(<pattern1> <e1>)
(<pattern2> <guard2> <e2>)
...
(<patternn> <guardn> <en>))
We could rewrite our absolute-value function using case
like this:
(defun abs (x)
(case (< x 0)
('true (- x))
(_ x)))
When the final pattern matched against is the "I-don't-care variable",4 the effect is the same as the final 'true
in the cond
form: if all else fails to match, the expression associated with the _
pattern is returned.
Function Argument Patterns
In our discussion of conditionals, we would be remiss in our duties if we did not bring up the topic of pattern-matching in function arguments, or in this case, patterns and guards.
Ordinarily in LFE you define a function as we have discussed, using the form
(defun <name> (<arguments>) <body>)
However, like Erlang, LFE supports pattern matching and guards in functions. The more general form of function definition is
(defun <name>
((<argpattern1>) <body1>)
((<argpattern2>) <guard2> <body2>)
...
((<argpatternn>) <guardn> <bodyn>))
We can rewrite our absolute-value function using a simple pattern and guards
(defun abs
((x) (when (> x 0)) x)
((x) (when (== x 0)) 0)
((x) (when (< x 0)) (- x)))
which of course could be consolidated to
(defun abs
((x) (when (< x 0)) (- x))
((x) x))
Note that in both definitions above our argument pattern is simply x
. We are not using the mechanics of pattern matching, per se, to implement our conditional logic. Rather, in this case we are taking advantage of the argument pattern's optional guard.
Logical Operators as Predicates
In addition to primitive predicates such as <
, =
, and >
, there are logical composition operations, which enable us to construct compound predicates. The three most frequently used are these:
(and <e1> ... <en>)
- The interpreter evaluates the expressions<e>
one at a time, in left-to-right order. If any<e>
evaluates tofalse
, the value of theand
expression isfalse
, and the rest of the<e>
's are not evaluated. If all<e>
's evaluate totrue
values, the value of theand
expression istrue
.(or <e1> ... <en>)
- The interpreter evaluates the expressions<e>
one at a time, in left-to-right order. If any<e>
evaluates to atrue
value, the value of theor
expression istrue
, and the rest of the<e>
's are not evaluated. If all<e>
's evaluate tofalse
, the value of theor
expression isfalse
.(not <e>)
- The value of anot
expression istrue
when the expression<e>
evaluates tofalse
, andfalse
otherwise.
Notice that and
and or
are special forms, not functions, because the subexpressions are not necessarily all evaluated. not
is an ordinary function.
As an example of how these are used, the condition that a number be in the range may be expressed as
(and (> x 5) (< x 10))
As another example, we can define a predicate to test whether one number is greater than or equal to another as
(defun gte (x y)
(or (> x y) (== x y)))
or alternatively as
(defun gte (x y)
(not (< x y)))
1. "Interpreted as either true or false" means this: In LFE, there are two distinguished values that are denoted bytrue
andfalse
. When the interpreter checks a predicate's value, if the result cannot be interpreted as eithertrue
orfalse
, an error is raised (forcond
andif
you will see anif_clause
exception). ↩
2.abs
also uses the "minus" operator-
, which, when used with a single operand, as in(- x)
, indicates negation. ↩
3. A minor difference betweenif
andcond
is that the<e>
part of eachcond
clause may be a sequence of expressions. If the corresponding<p>
is found to betrue
, the expressions<e>
are evaluated in sequence and the value of the final expression in the sequence is returned as the value of thecond
. In anif
expression, however, the<consequent>
and<alternative>
must be single expressions. ↩
4. The single underscore, anonymous variable, "blah", "don't care", or "throw-away" variable has a long history in computing. Many languages, including Prolog, C, Erlang, OCaml, Python, Ruby, etc., share a tradition of treating the underscore as a special variable; special in the sense that the value is not seen as being of any pertinent interest in the given context. In Prolog and Erlang, and thus LFE, the anonymous variable has a significant functional purpose: it is never bound to a value and as such can be used multiple times in pattern matching without throwing an error (which would happen in Prolog or Erlang if you tried to match multiple patterns with a regular, bound variable). ↩