pyfoma-morphology

name	pyfoma-morphology
description	Build and review complex morphological analyzers/generators in PyFoma with clear separation of lexd morphotactics and FST.re morphophonology, including robust handling of tagging, rule ordering, reduplication, root-pattern morphology, and regression testing.

pyfoma-morphology skill

This skill is for implementing or reviewing production-quality morphology grammars in PyFoma.

Use with:

• references/REFERENCE.md for PyFoma regex/rewrite details and design patterns.
• references/LEXD.md for full lexd syntax and semantics in this repository.

Output expectations

When asked to build or revise a grammar, produce:

1. Grammar code:
   • lexd + rule cascade when lexd is available
   • FST.re lexical transducer + rule cascade when lexd is unavailable
2. A minimal executable test snippet that verifies generation and analysis.
3. Short notes on rule ordering and known edge cases.
4. A brief generalization note:
   • productive pattern(s)
   • explicit exception list
   • why each lexical class marker is needed (if any)

Analyzer objectives

A morphological analyzer is not a memorized pair list. It should:

• encode productive inflection with general rules
• keep lexical listing for stems and true irregular exceptions
• map lexical strings with tags to clean orthographic outputs
• map orthographic outputs back to lexical analyses

Workflow

1. Define lexical contract before coding

Freeze these first:

• Tag inventory: <N>, <V>, <Past>, <Pl>, etc.
• Boundary symbols used during intermediate forms (often +).
• Intermediate morpheme policy:
  • prefer canonical morphs with boundaries (for example +s, +es)
  • default rule: do not use abstract markers like +PL
  • exception only if canonical morph representation is demonstrably impractical in the runtime, and the reason is documented
• Expected API usage:
  • generate(lexical) for generation
  • analyze(surface) for analysis
• Side contract:
  • lexical side may contain tags and temporary class markers
  • final surface side must contain only orthographic wordforms (no tags, no SG/PL markers, no +)

If this contract is unstable, delay rule-writing.

2. Build morphotactics with lexd

Use lexd.compile(...) for:

• Paradigm structure and affix sequencing
• Lexically conditioned alternation choices (via tags/selectors)
• Non-concatenative structure supported by this implementation (reduplication/root-pattern patterns)

Keep phonological alternation out of lexd unless it is true suppletion/irregular listing. If a stem-internal alternation is substantially clearer as a direct transducer over full stems, model that alternation in FST.re and keep lexd for class/tag routing.

Anti-pattern to avoid:

• Do not list productive pluralized outputs directly per noun in lexd.
• Do not encode singular/plural as literal output strings like catSG, catPL, etc.
• If multiple lemmas share behavior, model it once with a rule and (if needed) a lexical class marker.
• Do not create many ad hoc lexical class markers when the behavior can be expressed by orthographic/phonological context rules.
• Do not use one-off marker classes unless they are true exceptions and documented as such.

2.1 If lexd is unavailable

Fallback to FST.re without changing grammar design principles:

• Build a stem inventory FST.
• Build an inflection/tag transducer FST.
• Compose them into a lexical transducer.
• Keep productive alternations in rewrite rules.

Even in fallback mode:

• do not collapse into per-lemma hard-coding
• keep an explicit productive/default vs exception split

3. Validate morphotactics in isolation

Before adding rewrite rules:

• Test at least 10 positive lexical->surface pairs.
• Test at least 10 positive surface->lexical pairs.
• Test at least 5 negative forms.
• Verify morphotactic stage still contains expected intermediate markers (if any) and has not baked in full productive alternations.

Do not proceed to morphophonology until these pass.

4. Add rewrite rules incrementally

Write one process per rule using FST.re("$^rewrite(...)").

Required discipline:

• Keep each rule name descriptive.
• Add one comment example per rule.
• Preserve boundaries/tags until final cleanup.
• Prefer rules stated over natural symbol classes/contexts (e.g., vowel-final vs consonant-final), not rules tied to tiny hand-built categories.
• If a rule only handles one or two lexemes, move that behavior to an explicit exception path instead of pretending it is productive.
• Prefer rewriting against canonical intermediate morphs (e.g., +s, +es) rather than abstract grammatical placeholders when possible.
• If abstract placeholders are used temporarily during development, convert to canonical morph representation before final delivery unless a documented exception applies.

Compose strictly as a left-to-right cascade:

final = FST.re("$lexicon @ $rule1 @ $rule2 @ $rule3 @ $cleanup", fsts)

5. Test each rule after composition

After each new rule:

• Re-run small focused tests for the new phenomenon.
• Re-run previous regression cases.
• Verify both generate and analyze.

For overlapping matches, explicitly test default vs directed behavior:

• leftmost=True
• longest=True
• shortest=True

6. Add irregularity explicitly

Prefer separate lexicon paths for irregulars instead of forcing them into productive rules.

Example pattern:

• Productive stems in NounRoot + productive inflection.
• Irregular whole forms in IrregularNoun lexicon block.

Irregular-path exclusivity rule:

• Do not let an irregular lemma also flow through an unblocked productive path.
• Either keep irregular lemmas out of productive stem classes, or add selector/class tags (for example [irreg], [noplural]) that explicitly block the productive paradigm for those lemmas.

7. Final QA

Pass criteria:

• Stable tag scheme and readable grammar layout.
• No rule relies on symbols already deleted by earlier rules.
• No unexplained nondeterminism in expected deterministic cases.
• Regression set includes productive, irregular, and edge cases.
• Gold-set automaton checks are run for coverage and overgeneration: grammar @ gold and grammar @ ~gold, plus projection-set diffs if needed (gold - output(grammar), output(grammar) - gold).
• Surface-side hygiene checks pass:
  • no <...> tags in generated forms
  • no + boundary symbols in generated forms
  • no grammatical marker suffixes/prefixes like SG, PL in generated forms
• Overfitting checks pass:
  • productive rules are few and broad, not many micro-rules for tiny subsets
  • each lexical marker class is linguistically named and justified
  • exception handling is explicit and separated from productive rules
• Irregular exclusivity checks pass:
  • no unintended regular variants for irregular lemmas (unless explicitly intended)
• Intermediate representation checks pass:
  • no abstract inflection placeholders in the final grammar unless explicitly justified

Compact lexd guide (full details in references/LEXD.md)

Core sections:

• PATTERNS
• PATTERN Name
• LEXICON Name
• ALIAS Source Alias

Frequent operators/features:

• ?, *, +, |
• Grouping: ( ... )
• Anonymous lexicon: [ ... ]
• Selectors: [tag], [-tag], [|[a,b]], [^[a,b]]
• Columns: Root(2)
• Side markers: :Lex, Lex:
• Paired columns: X(1):X(2), X(1):Y(2)
• Optional segmented lexicon: Name?(1)
• Sieve expansion: <, >

Critical semantic points:

• Repeating the same lexicon in one sequence usually binds the same chosen entry.
• Use ALIAS when repeated positions should vary independently.
• Tag selectors constrain combinations but do not emit symbols.

Design-quality points:

• Lexical class markers are a last resort, not the first strategy.
• Prefer one productive rule plus a small exception list over many marker-specific rules.

Guardrails

• In FST.re, always quote multichar symbols: '<N>', '+', '{A}'.
• Treat wildcard . as advanced; avoid accidental use.
• Keep morphotactics and morphophonology separate.
• Prefer several small rules to one unreadable mega-rule.
• Never assume generator/analyzer output order is stable.

Minimal starter template

from pyfoma import FST, lexd

fsts = {}

fsts['lexicon'] = lexd.compile(r'''
PATTERNS
NounRoot NounInfl

LEXICON NounRoot
house
spy
church

LEXICON NounInfl
<N><Sg>:
<N><Pl>:+s
''')

fsts['y_to_ie'] = FST.re("$^rewrite(y:(ie) / _ '+' s)")
fsts['epenthesis'] = FST.re("$^rewrite('':e / (sh|ch|x|s|z) _ '+' s)")
fsts['cleanup'] = FST.re("$^rewrite('+':'')")

grammar = FST.re("$lexicon @ $y_to_ie @ $epenthesis @ $cleanup", fsts)

print(list(grammar.generate("spy<N><Pl>")))
print(list(grammar.analyze("churches")))