[R6RS] What are predicates?

[William D Clinger]

Mike wrote:
> Some of the list procedures we're adding accept predicates as
> arguments.  As per R5RS, a predicate is a procedure returning a
> boolean value, i.e. #t or #f.  However, procedures like `exists' could
> also handle any true value returned by the predicate.  Should they?
> (I'm thinking yes.)

That would be far superior to requiring them to raise
an exception if the predicate were to return something
other than a boolean (e.g. memq).

Will