PHI 12 - tiles & bottom-up zipper

12-tiles-and-bottom-up-zipper/12-tiles-and-bottom-up-zipper.md

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+## Summary
+
+The first step toward a tile-based editor will be a pure refactor (modulo fixing
+some bugs or smoothing out some small but awkward ux details along the way).
+
+This refactor will reorganize the Hazel codebase around two central concepts: a
+tile-based syntax and a bottom-up zipper based edit state.
+
+A tile-based syntax will generalize our existing `OpSeq`-based syntax to better
+support prefix/postfix unary operators and unifies current structural
+discrepancies across sorts (eg blocks/lines in expressions but not
+types/patterns).
+
+A bottom-up zipper based edit state, a departure from our current top-down
+zipper structure (see `ZExp`/`ZPat`/`ZTyp`), will enable vastly better reuse of
+type checking logic across our codebase, in particular the logic for propagating
+type-contextual information down the tree. A bottom-up zipper splits the program
+tree into a subject (what is currently focused) and a frame (everything else).
+Having a frame datatype allows us to write the type-contextual info propagation
+logic once and reuse as needed without having to write a new type-directed
+recursive traversal as we currently do.