package fix

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Module Fix.HashConsSource

HashCons offers support for setting up a hash-consed data type, that is, a data type whose values carry unique integer identifiers.

The type 'data cell describes a cell that carries a unique identifier id as well as a payload data.

This type is marked private, which means that the user has no direct way of allocating cells. Instead, the user must apply the functor Make (below) to obtain a function make which either allocates a fresh cell or returns an existing cell. The user is still allowed to read existing cells.

Sourcetype 'data cell = private {
  1. id : int;
  2. data : 'data;
}

Accessors.

Sourceval id : 'data cell -> int
Sourceval data : 'data cell -> 'data

Cells come with an equality test, a comparison function, and and a hash function. These functions exploit the cell's unique identifier only -- the data is ignored.

Wherever a module of signature HashedType with type t = foo cell is expected, the module HashCons can be supplied. This holds regardless of the type foo.

Sourceval equal : 'data cell -> 'data cell -> bool
Sourceval compare : 'data cell -> 'data cell -> int
Sourceval hash : 'data cell -> int

A hash-consing service allocates uniquely-numbered cells for data. The smart constructor make either allocates a fresh cell or returns an existing cell, as appropriate.

Sourcemodule type SERVICE = sig ... end

The functor Make expects a type data for which a memoizer exists, and produces a hash-consing service for it.

Sourcemodule Make (M : sig ... end) : SERVICE with type data = M.key

ForHashedType is a special case of Make where it suffices to pass a hashed type T as an argument. A hash table is used to hold the memoization table.

ForHashedTypeWeak is a special case of Make where it suffices to pass a hashed type T as an argument. A weak hash table is used to hold the memoization table.