A type is a set of values, plus an associated set of operations
valid on those values.
Types are useful for catching errors ("type-checking"), documenting
the programmer’s intent, and to help the compiler generate better code.
Types in some languages (such as C) appear in programs,
but do not exist at run-time. In such languages, all type-checking
is done at compile-time. Other languages (such as standard Scheme)
do not have types as such, but they have predicates, which
allow you to check if a value is a member of certain sets; also,
the primitive functions will check at run-time if the arguments
are members of the allowed sets. Other languages, including Java
and Common Lisp, provide a combination: Types may be used as specifiers
to guide the compiler, but also exist as actual run-time values.
In Java, for each class, there is a corresponding
run-time object, as well as an associated type (the set of values
of that class, plus its sub-classes, plus
Kawa, like Java, has first-class types, that is types exist as
objects you can pass around at run-time. For each Java type,
there is a corresponding Kawa type (but not necessarily vice
versa). It would be nice if we could represent run-time
type values using
java.lang.Class objects, but unfortunately
that does not work very well. One reason is that we need
to be able to refer to types and classes that do not exist yet,
because we are in the processing of compiling them. Another
reason is that we want to be able to distinguish between different
types that are implemented using the same Java class.
Various Kawa constructs require or allow a type to be specified. Those specifications consist of type expressions, and a type expression is evaluated to yield a type value. The current Kawa compiler is rather simple-minded, and in many places only allows simple types that the compiler can evaluate at compile-time. More specifically, it only allows simple type names that map to primitive Java types or Java classes.