Global key-value metadata storage for Scryer Prolog

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A global environment/context management system for Scryer Prolog providing key-value metadata storage one level of abstraction above the blackboard.

This library provides a clean interface for managing global state in Prolog programs using association trees (AVL trees) backed by the blackboard. It offers:

Global persistent storage - Changes persist across backtracking
Backtrackable local storage - Changes roll back on backtracking
Type-safe reified predicates - Use if_/3 for conditional logic without cuts
Once-only initialization - Prevent accidental double initialization
Key existence checking - Efficiently check for key presence

bakage.pl install environment

Copy environment.pl to your project's library directory or ensure it's in your Scryer Prolog load path.

library(reif) - Reified predicates for conditional logic
library(assoc) - Association trees (AVL trees)
library(iso_ext) - ISO extensions including blackboard predicates

?- use_module(library(environment)). % Set global values (persistent across backtracking) ?- env_set_global(username, 'Alice'). true. ?- env_set_global(counter, 42). true. % Retrieve values ?- env_key_val(username, User). User = 'Alice'. % Check if key exists (reified) ?- env_check_flag_t(username, T). T = true. % Set local value (backtrackable) ?- env_set_local(temp, 'temporary'), env_key_val(temp, V). V = 'temporary'. % Initialize once (prevents double initialization) ?- env_set_global_once(config_loaded, true). true. ?- env_set_global_once(config_loaded, false). ERROR: runtime_error(env_set_global_once)

environment(-Environment)

Retrieve the current global environment as an association tree.

Replace the entire global environment (persistent).

Replace the entire global environment (backtrackable).

env_set_global(+Key, +Value)

Set a key-value pair globally (persistent across backtracking).

env_set_global_flag(+Key)

Set a flag (key with value true) globally.

env_set_global_once(+Key, +Value)

Set a key-value pair only if it doesn't exist. If it exists with a different value, throws an error.

env_set_global_flag_once(+Key)

Set a flag only once. Throws error if already set with different value.

env_set_local(+Key, +Value)

Set a key-value pair locally (backtrackable).

Set a flag locally (backtrackable).

env_key_val(+Key, ?Value)

Retrieve value for a key. Throws key_error if key doesn't exist.

?- env_set_global(name, 'Bob'), env_key_val(name, N). N = 'Bob'.

env_key_val_notfound(+Key, ?Value, +NotFound)

Retrieve value for a key, or unify with NotFound if key doesn't exist.

?- env_key_val_notfound(missing_key, V, default). V = default.

Check if a flag exists (deterministic success/failure).

env_check_flag_t(+Key, ?Truth)

Reified version: unifies Truth with true or false.

?- env_set_global_flag(debug), env_check_flag_t(debug, T1), env_check_flag_t(missing, T2). T1 = true, T2 = false.

env_check_key_val(+Key, +Value)

Check if key exists with specific value (deterministic).

env_check_key_val_t(+Key, +Value, ?Truth)

Reified version of key-value check.

env_check_key_value_t(+Key, +Value, ?Truth)

Alias for env_check_key_val_t/3.

Assert that a flag exists (throws error if missing).

Remove a key from the environment (persistent).

Remove a key from the environment (backtrackable).

% Initialize configuration once init_config :- env_set_global_once(db_host, 'localhost'), env_set_global_once(db_port, 5432), env_set_global_flag_once(config_loaded). % Access configuration get_db_connection(Host, Port) :- env_key_val(db_host, Host), env_key_val(db_port, Port).

% Enable/disable features enable_feature(Feature) :- env_set_global_flag(Feature). disable_feature(Feature) :- env_remove(Feature). % Check if feature is enabled feature_enabled(Feature) :- env_check_flag(Feature). % Conditional execution maybe_log(Message) :- env_check_flag_t(logging_enabled, T), if_(T=true, format('LOG: ~w~n', [Message]), true ).

% Use local state that backtracks process_with_context(Data, Result) :- env_set_local(current_data, Data), process_step_1, process_step_2, env_key_val(result, Result).

% Increment counter increment_counter(Name) :- env_key_val_notfound(Name, Current, 0), Next is Current + 1, env_set_global(Name, Next). % Usage ?- increment_counter(requests), increment_counter(requests). true. ?- env_key_val(requests, Count). Count = 2.

The environment is initialized exactly once using term_expansion/2. The first time the module is loaded, it creates an empty association tree and stores it in the blackboard under global_context.

This library extensively uses reified predicates from library(reif) for conditional logic without cuts. This enables pure, declarative programming with backtracking-friendly conditionals.

All operations are O(log n) due to the underlying AVL tree implementation
Key lookups are efficient even with thousands of entries
Reified key existence checking avoids expensive exception handling

BSD-2-Clause (compatible with Scryer Prolog standard libraries)

Jay

0.1.0

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