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55 lines
1.6 KiB
Erlang
55 lines
1.6 KiB
Erlang
-module(projection).
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-export([new/2, new/3, fold_activity/2, replay/2,
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name/1, state/1, fold_fn/1]).
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%% Pure-functional projection driver per design §10.
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%%
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%% A projection is a property list:
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%% [{name, atom}, {state, term}, {fold, fun}]
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%%
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%% The fold function is `fun (Activity, State) -> NewState`. v1
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%% uses Erlang funs as the fold body — the genesis bundle's SX
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%% `:fold` bodies are stored as binaries; an SX-source eval
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%% bridge will plug them into the same projection record once
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%% it lands (Step 7d). For now, callers supply Erlang funs
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%% directly when constructing a projection.
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%%
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%% `replay/2` is the cold-start primitive: fold an activity
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%% list (e.g. `log:entries/1`) through the projection from its
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%% initial state.
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new(Name, InitialState) ->
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new(Name, InitialState, fun (_Activity, S) -> S end).
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new(Name, InitialState, FoldFn) ->
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[{name, Name}, {state, InitialState}, {fold, FoldFn}].
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fold_activity(Proj, Activity) ->
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Fn = fold_fn(Proj),
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S0 = state(Proj),
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S1 = Fn(Activity, S0),
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set_field(state, S1, Proj).
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replay(Proj, Activities) ->
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fold_each(Proj, Activities).
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fold_each(Proj, []) -> Proj;
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fold_each(Proj, [A | Rest]) ->
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fold_each(fold_activity(Proj, A), Rest).
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%% Accessors
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name(Proj) -> field(name, Proj).
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state(Proj) -> field(state, Proj).
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fold_fn(Proj) -> field(fold, Proj).
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%% Internal
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field(K, [{K, V} | _]) -> V;
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field(K, [_ | Rest]) -> field(K, Rest);
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field(_, []) -> erlang:error(badkey).
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set_field(K, V, [{K, _} | Rest]) -> [{K, V} | Rest];
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set_field(K, V, [P | Rest]) -> [P | set_field(K, V, Rest)];
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set_field(K, V, []) -> [{K, V}].
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