An identifier can name either a type of syntax or a location
where a value can be stored. An identifier that names a
type of syntax is called a syntactic keyword
(informally called a macro), and is said to be
bound to a transformer for that syntax. An identifier that
names a location is called a variable and is said to be bound
to that location. The set of all visible bindings in effect at
some point in a program is known as the environment in
effect at that point. The value stored in the location to
which a variable is bound is called the variable’s value.
By abuse of terminology, the variable is sometimes said
to name the value or to be bound to the value. This is
not quite accurate, but confusion rarely results from this
practice.
Certain expression types are used to create new kinds of
syntax and to bind syntactic keywords to those new syntaxes,
while other expression types create new locations
and bind variables to those locations. These expression
types are called binding constructs.
Those that bind syntactic keywords are discussed in Macros.
The most fundamental of the variable binding constructs is the
lambda expression,
because all other variable binding constructs
can be explained in terms of lambda expressions.
Other binding constructs include the define family,
and the let family.
Scheme is a language with block structure. To each place
where an identifier is bound in a program there corresponds
a region of the program text within which the binding is visible.
The region is determined by the particular binding construct that
establishes the binding; if the binding is
established by a lambda expression, for example, then its
region is the entire lambda expression. Every mention of
an identifier refers to the binding of the identifier that established
the innermost of the regions containing the use.
If there is no binding of the identifier whose region contains the use,
then the use refers to the binding for the
variable in the global environment, if any;
if there is no binding for the identifier, it is said to be unbound.
The usual way to bind variables is to match an incoming
value against a pattern. The pattern contains variables
that are bound to some value derived from the value.
(! [x::double y::double] (some-expression))
In the above example, the pattern [x::double y::double]
is matched against the incoming value that results from
evaluating (some-expression).
That value is required to be a two-element sequence.
Then the sub-pattern x::double is matched against
element 0 of the sequence, which means it is coerced to a double
and then the coerced value is matched against the sub-pattern x
(which trivially succeeds). Similarly, y::double is matched
against element 1.
The syntax of patterns is a work-in-progress. (The focus until now
has been in designing and implementing how patterns work in general,
rather than the details of the pattern syntax.)
This is how the specific patterns work:
-
identifier
This is the simplest and most common form of pattern.
The identifier is bound to a new variable
that is initialized to the incoming value.
-
_
This pattern just discards the incoming value.
It is equivalent to a unique otherwise-unused identifier.
-
pattern-literal
Matches if the value is equal? to the pattern-literal.
-
’datum
Matches if the value is equal? to the quoted datum.
-
pattern :: type
The incoming value is coerced to a value of the specified type,
and then the coerced value is matched against the sub-pattern.
Most commonly the sub-pattern is a plain identifier,
so the latter match is trivial.
-
[ lpattern* ]
The incoming value must be a sequence (a list, vector or similar).
In the case where each sub-pattern is a plain pattern,
then the number of sub-patterns must match the size of the sequence, and
each sub-pattern is matched against the corresponding element of the sequence.
More generally, each sub-pattern may match zero or more consequtive
elements of the incoming sequence.
-
#!if expression
No incoming value is used. Instead the expression is evaluated.
If the result is true, matching succeeds (so far);
otherwise the match fails.
This form is called a guard.
-
@ pattern
-
A splice pattern may match multiple (zero or more) elements of a sequence.
The pattern is matched against the resulting sub-sequence.
(! [x @r] [2 3 5 7 11])
This binds x to 2 and r to [3 5 7 11].
-
pattern ...
Similar to @pattern in that it matches
multiple elements of a sequence.
However, each individual element is matched
against the pattern, rather than the elements as a sequence.
This is a repeat pattern.
