A type is a named value for a set
of objects with related properties.
For example
vector is the type for standard Scheme vectors.
You can use a type to specify that a variable can only have
values of the specified types:
#|kawa:5|# (define v ::vector #(3 4 5)) #|kawa:6|# v #(3 4 5) #|kawa:7|# (set! v 12) /dev/stdin:7:1: warning - cannot convert literal (of type gnu.math.IntNum) to vector Value (12) for variable 'v' has wrong type (gnu.math.IntNum) (gnu.math.IntNum cannot be cast to gnu.lists.FVector) at atInteractiveLevel$7.run(stdin:7) at gnu.expr.ModuleExp.evalModule(ModuleExp.java:302) at kawa.Shell.run(Shell.java:275) at kawa.Shell.run(Shell.java:186) at kawa.Shell.run(Shell.java:167) at kawa.repl.main(repl.java:870) Caused by: java.lang.ClassCastException: gnu.math.IntNum cannot be cast to gnu.lists.FVector ... 6 more
Using a type specification catches errors, and matches your programs more readable. It also can allow the Kawa compiler to generate code that runs faster.
You can use a type to check that a value is an instance of the type,
using either the
instance? function:
(instance? #(3 4 5) vector) ⇒ #t (instance? '(3 4 5) vector) ⇒ #f
As a convenience, you can use a type-name followed by a “
?”:
(
type?
val) == (instance?
val
type)
You can “call” a type as if it were a function. That constructs a new instance of the type.
((EXAMPLE NEEDED))
A fully-qualified Java class is a type name. So are the names of Java primitive types. So are Java array types. ((EXAMPLES NEEDED))
A type is true run-time value:
(define mytypes (list vector list string)) (instance? #(3 4 5) (car mytypes) ⇒ #t
The
define-alias form is useful for defining shorter names
for types, like a generalization of Java’s
import statement:
(define-alias jframe javax.swing.JFrame)