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upcast.sx: register a pure (event -> event) upcaster per type in an immutable registry; read-upcast/project-upcast lift legacy events to the current shape on read so projections see one shape (no version branching, no history rewrite). upcast-data helper merges new :data fields. 171/171. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
45 lines
1.5 KiB
Plaintext
45 lines
1.5 KiB
Plaintext
; persist/upcast — event schema evolution. An append-only log keeps events
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; forever, so old events have old shapes. Rather than migrate stored data (you
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; can't rewrite history) or branch every projection on version, register an
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; upcaster per event type: a pure (event -> event) that lifts an old event to
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; the current shape. Reads pass through the registry so projections see ONE
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; shape. The registry is an immutable dict the consumer threads (no global
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; mutable state). Requires: lib/persist/event.sx, lib/persist/log.sx.
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(define persist/upcasters (fn () {}))
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(define persist/register-upcaster (fn (reg type fn) (assoc reg type fn)))
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; apply the registered upcaster for an event's type, or pass it through unchanged
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(define
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persist/upcast
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(fn
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(reg e)
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(let ((f (get reg (persist/event-type e)))) (if f (f e) e))))
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; read a stream with every event lifted to current shape
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(define
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persist/read-upcast
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(fn
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(b stream reg)
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(map (fn (e) (persist/upcast reg e)) (persist/read b stream))))
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; project over upcasted events — projections never see a legacy shape
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(define
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persist/project-upcast
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(fn
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(b stream reg step seed)
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(reduce step seed (persist/read-upcast b stream reg))))
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; helper: upcast an event's :data by merging in/overriding fields, keeping the
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; record's stream/seq/type/at. Common upcaster body.
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(define
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persist/upcast-data
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(fn
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(e new-data)
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(persist/event
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(persist/event-stream e)
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(persist/event-seq e)
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(persist/event-type e)
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(persist/event-at e)
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(merge (persist/event-data e) new-data))))
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