[plt-scheme] define-promise-type and let-promise-type

Attached a module that exports delay, lazy, force and promise?, but in addition the syntaxes: define-promise-type and let-promise-type. The latter two define cq locally bind distinct types of promises. A force of a promise of some type does not force promises of any other type. The module is based on that currently used in PLT Scheme.

Syntax define-promise-type would allow me to implement srfi 41 for PLT fully compliant to the srfi 45 document without duplicating srfi-45.

Your comments are welcome, of course.

Best wishes, Jos





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