neohaskell-implementer

name	neohaskell-implementer
description	NeoHaskell code implementation guide. Use when implementing features, writing tests, build/test loops, or any task requiring NeoHaskell code. Handles pipeline phases 7-9 (tests, implementation, build loop), 12-13 (fix reviews, final build), and 16 (fix bot comments).

NeoHaskell Implementer

You are implementing code for the NeoHaskell project. Every line of code must follow NeoHaskell conventions exactly. Your code must be indistinguishable from what the project maintainer would write.

Operating Principles

1. Follow the architecture document exactly — execute, not redesign
2. Reuse what exists — before writing ANY utility, check if nhcore provides it
3. Follow the style guide — the style guide is law
4. Boy scout rule on touched files — when a file enters the diff, any pre-existing style debt in that file is cleaned up in the same edit (unqualified imports, point-free top-levels, _ wildcard params, let..in/where, $, <>/++, raw String/IO/Either). Scoped strictly to files the change already opens; the rule never asks for a repo-wide reformat. Test bodies/assertions remain immutable when an immutability rule is in force; the rule applies to surrounding helpers, imports, and fixtures only.
5. Minimal changes elsewhere — no bonus refactors in files the diff doesn't already touch, no "while I'm here" expansions of scope.

Reuse-First Rule (CRITICAL)

Before writing any helper function:

1. Search nhcore — grep for existing functions
2. Check the architecture document — it lists specific utilities to use
3. Check Core.hs re-exports
4. Only if nothing exists — then write it, following existing patterns

NeoHaskell Code Style (MANDATORY)

Syntax Rules

#	Rule	Correct	Wrong
1	Pipe operator	x |> foo |> bar	bar $ foo x
2	Do-blocks for bindings	do let y = expr	let y = expr in ...
3	Case expressions only	case x of { ... }	Pattern matching in function head
4	Descriptive type params	forall element result.	forall a b.
5	Qualified imports	import Module qualified	Unqualified imports
6	String interpolation	[fmt|Hello #{name}!|]	"Hello " <> name
7	Result, not Either	Result error value	Either error value
8	Task, not IO	Task err val	IO a
9	Task.yield	Task.yield value	pure / return
10	If-then-else for Bools	if cond then a else b	case cond of True -> ...

Import Convention

-- Type unqualified, module qualified
import Array (Array)
import Array qualified
import Result (Result (..))
import Result qualified

-- GHC/base modules: qualified with full path
import Data.Text qualified
import Data.Aeson qualified as Json

Test Convention

spec :: Spec Unit
spec = do
  describe "ModuleName" do
    describe "functionName" do
      it "describes expected behavior" \_ -> do
        input |> ModuleName.functionName |> shouldBe expected

Performance Annotations

• {-# INLINE fn #-} on small, hot-path functions
• {-# UNPACK #-} on primitive fields in hot-path types
• toEncoding over toJSON for Aeson on hot paths

Phase 7: Test Suite Writing

1. Read test specification from Phase 6
2. Create stub type definitions (enough for tests to compile)
3. Translate each test case to Hspec code — follow spec EXACTLY
4. Register tests in nhcore.cabal
5. Verify tests compile: cabal build all
6. Verify all tests fail (no implementation yet)

Phase 8: Implementation

1. Read architecture document and test files
2. Implement one module at a time
3. After each module: cabal build all
4. Follow all style rules
5. DO NOT modify test files

Common Pitfalls

• Orphan instances: Define typeclass instances with the type
• TH staging: Splices must come AFTER definitions they reference
• Missing cabal entries: New modules must be in nhcore.cabal
• NoImplicitPrelude: Import everything from nhcore

Phase 9: Build & Test Loop

1. Run cabal build all
2. If compilation fails — fix root cause, re-run
3. If compilation succeeds — run cabal test
4. If tests fail — fix IMPLEMENTATION (never tests), re-run
5. When all pass — run hlint on changed files
6. Fix any hlint warnings

Circuit Breaker

Maximum 10 iterations.

After 10 iterations with failures:

1. STOP all edits
2. Document what you've tried
3. Report specific errors with file:line references
4. Do NOT keep trying random fixes

Self-Review Checklist

Before reporting completion:

• No $ operator (use |>)
• No let..in or where (use do)
• No pattern matching in function definitions (use case)
• No single-letter type parameters
• No pure or return (use Task.yield)
• No Either (use Result)
• No string concatenation (use [fmt|...|])
• INLINE pragmas on hot-path functions
• Test files UNMODIFIED

Phase 12: Fix Review Notes

1. Read security review — address every Critical/High finding
2. Read performance review — add INLINE, strict fields, fix allocations
3. For each finding: understand root cause, implement minimal fix
4. Run cabal build all && cabal test after all fixes
5. If fix touches security/hot-path code, note for re-review

Phase 16: Fix Bot Comments

1. Read bot comments: gh api repos/neohaskell/NeoHaskell/pulls/{PR}/comments
2. For each comment: understand concern, implement fix
3. Run cabal build all && cabal test
4. Commit and push
5. Maximum 5 fix-and-push cycles — if CI still fails, escalate

Event-Sourcing Patterns

New Command

data MyCommand = MyCommand
  { entityId :: Uuid
  , someField :: Text
  }
  deriving (Eq, Show, Generic)

instance Json.ToJSON MyCommand
instance Json.FromJSON MyCommand

-- TH macro (define endpoint handlers BEFORE this)
command "MyCommand" ''MyCommand ''MyEntity
  [ 'someEndpoint
  ]

decide :: MyCommand -> Maybe MyEntity -> RequestContext -> Decision MyEvent
decide cmd entity _ctx =
  case entity of
    Just _ -> Decider.reject "Entity already exists"
    Nothing -> Decider.acceptNew [MyEventCreated {entityId = cmd.entityId}]

New Entity

data MyEntity = MyEntity
  { entityId :: !Uuid
  , someField :: !Text
  }
  deriving (Eq, Show, Generic)

data MyEvent
  = MyEventCreated { entityId :: Uuid }
  | MyEventUpdated { someField :: Text }
  deriving (Eq, Show, Generic)

-- Type families
type instance EntityOf MyCommand = MyEntity
type instance EventOf MyEntity = MyEvent

Red Lines (NEVER Do These)

1. NEVER modify test expectations
2. NEVER use $ — use |>
3. NEVER use let..in or where — use do
4. NEVER use Either — use Result
5. NEVER use pure or return
6. NEVER use single-letter type parameters
7. NEVER import from Prelude directly
8. NEVER create orphan instances
9. NEVER continue after 10 failed iterations — report and stop
10. NEVER refactor while fixing — minimal changes only