Dry And Reuse

dry-and-reuse — search before you build, consolidate before you clone

Purpose: kill the duplication that gives AI code its short shelf life. GitClear's 211M-line study found copy-pasted code now exceeds refactored code for the first time ever, and duplicated blocks rose 8× — because agents reflexively add and almost never move/merge. Duplicated blocks carry 15–50% more defects. The senior move is to find what exists and reuse it.

DRY is about knowledge, not text

The real rule (Pragmatic Programmer): every piece of knowledge has a single, authoritative representation. Two consequences agents get wrong in both directions:

• Two blocks that look alike but encode different decisions are not a DRY violation — don't force them into one abstraction (that's coupling unrelated things).
• One business rule expressed in three places is a violation even if the code looks different each time — that's the dangerous kind, because the copies drift.

Reflex card

1. Search before you build. Before writing a helper/util/type/constant, grep the codebase (and the stdlib, and existing deps) for one that already does it. Reuse beats rewrite, every time.
2. Consolidate, don't clone. About to copy-paste a block? Extract it to one place and call it twice instead. Copy-paste is a debt you pay at every future change.
3. Move/merge, don't only add. The healthiest diffs delete and relocate code as well as adding it. If your change is pure addition and you suspect overlap, look harder. The refactored-to-duplicated ratio is the #1 quality signal.
4. One rule, one home. A validation rule, a tax calculation, a status enum, a magic constant — define it once and import it. Never let the same knowledge live in two files.
5. Reuse the stdlib first. Don't hand-roll what itertools/lodash/slices/Intl already gives you. Reinventing stdlib is DRY slop and a bug farm.
6. But don't abstract incidental similarity. If pulling two look-alikes together requires flags and special-cases to keep them apart, they were different rules — leave them. (Hand the judgment to craft:right-sized-design.)

Red flags

• A new function whose body you've seen elsewhere in the repo.
• The same constant / regex / URL / status string in 2+ files.
• A block copy-pasted and then lightly edited (the most defect-prone pattern).
• The same business rule (discount, tax, permission check) computed in several places.
• A util that re-implements groupBy, debounce, chunk, deepEqual, date math, etc.
• A "v2" of a function sitting next to "v1" with 90% overlap.
• A diff that only ever adds, never moves or deletes, across a large change.

The procedure

1. Before writing: search by intent (what it does), not just by name — agents miss existing helpers because they searched for the wrong noun. Grep for the operation, the constant's value, the rule's keywords.
2. On a hit: reuse it. If it's almost right, extend the existing one (carefully, without breaking its other callers) rather than forking a copy.
3. On real duplication found in review: extract to a single home (hand the mechanics to refactoring-patterns: Extract Function, Pull Up Method, Extract Class). Place the single source where both callers can reach it without creating a bad dependency.
4. Confirm it's knowledge duplication, not coincidence, before merging — see the "incidental similarity" caveat above.

Inline example

# ❌ Agent clones the rule into a third place
def can_edit(user, doc):
    return user.role == "admin" or user.id == doc.owner_id   # rule copy #3

# ✅ One authoritative home, reused
# permissions.py
def can_edit(user, doc) -> bool:
    """Single source of truth for edit permission."""
    return user.is_admin or user.id == doc.owner_id
# everywhere else: from permissions import can_edit

When the rule changes (say, editors can also edit), you change it once — not hunt three drifted copies.

Critique mode

Scan the diff for repeated logic, duplicated constants, and reinvented stdlib/existing utilities. For each: is this knowledge duplication (consolidate) or incidental similarity (leave it)? Tag [dry-and-reuse · duplicate-knowledge|reinvented-util · SEV]. Cross-check suspected structural duplication with dependency-analyzer.

References

• DRY-as-knowledge: ../../references/PRINCIPLES.md §A
• Duplicate Code / Repetition: ../../references/SMELLS.md
• The GitClear data: ../../references/EVIDENCE.md
• Rules 3 & 4: ../../RULES.md