.TH types 9 "September 7, 1994"
.SH NAME
Specifying type information in 
.I Progol.
.SH DESCRIPTION
.B
Types have to be specified for every argument of all predicates
to be used in constructing a hypothesis. This specification is
done by a
.I mode
declaration.
Type definitions can either be built-in to Prolog (for example,
.I int/1
or 
.I float/1
or unary predicates in background knowledge. 
In addition to most standard Prolog built-in definitions
.I
Progol
provides the following that could be of
use in defining types:
.TP
.B any(X)
any/1 is true for any term X.
.TP
.B float(X)
float/1 is true if X is a floating point number.
.TP
.B int(X)
int/1 is true if X is an integer.
.TP
.B nat(X)
nat/1 succeeds when X is a natural number.
.TP
.B constant(X)
constant/1 succeeds when X is a constant.
.SH EXAMPLES
Here is a type definition for a list of integers:

.DS 3 1
list([]).

list([H|T]):-

	int(H),

	list(T).
.SH "SEE ALSO"
mode/2