fold Projection

In One Sentence

The pure function that defines “how to compute a view from an event log” is the fold; the view it produces is the Projection.

Breaking It Down

• fold (fold/reduce): accumulates events one by one into a result. Input = all events, output = a derived state, a pure function with no side effects.
• Projection: the view produced by a fold, such as “the current conversation message list”, “the Goal state”, or “the session title” — each is one projection.

Core Philosophy

Event log (the single Source of Truth)
   ├─ fold → Projection A (message list)
   ├─ fold → Projection B (Goal state)
   └─ fold → Projection C (session title)

In Dsh-Go

1proj := session.FoldAll(sl.Events())
2for _, m := range proj.Messages {
3    fmt.Printf("[%s] %s\n", m.Role, m.Content)
4}

• FoldAll folds the entire log into a SessionProjection (containing the Messages / Goal / Todo / PlanMode sub-projections)
• Incremental fold (H04): instead of recomputing on every read, it is maintained incrementally in O(N)

Performance Data

Source Reference

• pkg/session/fold.go — the projection function family
• pkg/session/incremental.go — incremental projection (H04)
• Runnable example: examples/tutorial step 2

Next Steps

→ Learn the Goal State Machine: how an Agent plans and drives execution