remember

This Racket library provides a compile-time memoize feature. It allows remembering a value with (remember-write! 'category 'value). In subsequent compilations, (get-remembered 'category) will return a set of all previously-remembered values.

Installation

raco pkg install remember

Example use case: the phc-adt library

This library is used to implement "interned" structure and constructor types in the phc-adt library. The phc-adt library needs to know the set of all structure and constructor types used in the program, and uses remember to automatically memoize structure descriptors and constructor names.

When the structure macro defined in structure.hl.rkt encounters an unknown list of field names, it uses the remember library to append the tuple of field names to a user-specified file. That file is loaded in subsequent compilations, so that the tuple of fields is known to phc-adt.

The memoized descriptors are used to know all possible structs that can contain a field with the desired name when accessing it with (get instance field-name). The get macro can then retrieve the field's value using the right accessor (for example (struct123-fieldname instance)). Knowing all existing structures allows get to perform some kind of dynamic dispatch to obtain the appropriate accessor, for example using a cond which tests for all possible types.

The constructor macro defined in constructor.hl.rkt works in the same way, but remembers the name of the constructor's tag instead of field names. The memoization feature is used so that all uses of a constructor with a given name are equivalent, across all files.