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project.sx: projection state {:value :seq}; persist/project folds the whole
stream, persist/project-resume folds only the tail so read models update
incrementally. Pure step (value event)->value. 37/37.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
31 lines
1.2 KiB
Plaintext
31 lines
1.2 KiB
Plaintext
; persist/project — a projection folds a stream's events into a read model.
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; A projection state is {:value v :seq s} where s is the last seq folded in,
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; so a projection can resume incrementally from where it left off (replay only
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; the tail). step : (value event) -> value. Determinism: step must be pure —
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; time lives on the event (event-at), never a clock here.
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; Requires: lib/persist/event.sx, lib/persist/log.sx.
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; fold the tail (events with seq > prior's seq) onto a prior projection state
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(define
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persist/project-resume
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(fn
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(b stream step prior)
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(let
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((tail (persist/read-from b stream (+ 1 (get prior :seq)))))
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(reduce (fn (acc e) {:value (step (get acc :value) e) :seq (persist/event-seq e)}) prior tail))))
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; project the whole stream from seed
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(define
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persist/project
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(fn (b stream step seed) (persist/project-resume b stream step {:value seed :seq 0})))
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(define persist/project-value (fn (p) (get p :value)))
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(define persist/project-seq (fn (p) (get p :seq)))
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; convenience: project and return just the value
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(define
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persist/project-fold
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(fn
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(b stream step seed)
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(persist/project-value (persist/project b stream step seed))))
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