Elamur Groupantsmath-understand-pratt
The groupant beside math/src/understand.rs in the Mova repository: what the code computes, what it never does, how to go back from its outputs, its invariants, cost and checks.
Facts
Implementation material. A Mova code model describes this code at one source revision: partial models, finite checks and their stated scope. It is not a reviewed general exposition, and its labels are shown as written, never raised.
Components
| component | label | where |
|---|---|---|
| approximant | partial | approximant |
| ast | partial | ../syntax.json |
| descriptant | present | descriptant |
| invariant | partial | invariant |
| tests | executed bounded checks | tests |
Obernants
The typed relations around the forward model: the behaviour complement (what the code never does), the reverse reconstruction (every input compatible with an output) and specialised views.
| id | type | label | where |
|---|---|---|---|
behavior_complement | behavior_complement | partial source derived model | behavior_complement |
reverse_reconstruction | reverse_reconstruction | partial source derived model | reverse_reconstruction |
unary_power_order | unary_power_order | partial source derived model | unary_power_order |
of_dispatch | of_dispatch | partial source derived model | of_dispatch |
Model text
descriptant
math-understand-pratt
expr(min) parses unary first; +,- have precedence1, ,/2, power3 right-associative, square/cube4 postfix. For a nonpercent lhs and percent rhs under +/-, construct lhs(1 +/- rhs).
Epoch 2026-10-31: replaced duplicate catalog sentences with concrete exclusions, compatible-antecedent examples and two distinct scoped behavioral views. Source and prior execution evidence are unchanged; this revision ran no tests.
approximant
Scoped forward relation
expr(min) parses unary first; +,- have precedence1, ,/2, power3 right-associative, square/cube4 postfix. For a nonpercent lhs and percent rhs under +/-, construct lhs(1 +/- rhs).
This source-derived relation is partial; no executable percentage surrogate or replacement is supplied.
invariant
Scoped domain and invariants
Successful consumption advances cursor; parenthesized parsing consumes the matching RParen. Complete-stream checking belongs to chain, not expr alone.
Cost
Token traversal is linear for the grammar, with recursion proportional to nesting/operator association. Expr allocation and downstream exact evaluation are separate costs.
models
behavior_complement
A missing operand, unmatched closing/opening parenthesis, or fraction word without of cannot yield successful parse on that path. Unary-first parsing excludes treating -2^2 as -(2^2) in this grammar.
reverse_reconstruction
Extra parentheses around a value can yield the same Expr. Both percent tokens and equivalent explicit multiplication can evaluate alike while producing different trees; value equality does not recover token form or the rewrite path.
unary_power_order
expr calls unary before its operator loop, so unary minus becomes part of the power base. Power RHS uses min=3, while left-associative operators recurse at prec+1.
of_dispatch
Frac requires Of; a slash-form nonpercent Val followed by Of builds multiplication; a percent Val followed by Of builds PctOf; ordinary nonpercent Val followed by Of errors.
tests
Scoped evidence
Precedence, right-associative power, percentages, slash-fraction multiplication and malformed parentheses/fraction.
Named actual source fixtures: understand::wave28::{pratt_precedence_and_percent_rewrite}.
Parent tests.md records commands and paired profiles. No additional coverage inferred from parent fixtures outside this byte range; source-scoped numeric/parse properties remain partial.
This page in Markdown → https://md.groupants.elamur.ai/free/en/mova/math/src/understand.rs.groupant/pratt.groupant/