neohaskell-style-guide

name: neohaskell-style-guide description: NeoHaskell coding style reference and enforcement rules. Load when writing, reviewing, or modifying any Haskell code in the NeoHaskell project. Triggers on 'NeoHaskell style', 'NeoHaskell conventions', 'how to write NeoHaskell', 'code style', 'style guide'.

NeoHaskell Style Guide

This is the authoritative coding style reference for the NeoHaskell project. NeoHaskell is NOT standard Haskell — it has its own conventions that differ significantly. Violations of these rules are hard failures.

Boy scout rule (load-bearing)

When this skill is loaded to write or modify Haskell code, every file the diff opens for writing is fair game for cleaning up pre-existing style debt. If a touched file already has unqualified import Module (helper), point-free top-level bodies, _ wildcard parameters, let..in/where, $, <>/++ for strings, or raw String/IO/Either, fix those violations in the same edit — not in a follow-up PR.

Scope: only files the current change already includes. The rule never asks for a repo-wide reformat.

Test bodies and assertions remain immutable when an immutability rule is in force (e.g. phase 10 of the feature pipeline); the boy scout rule applies to surrounding helpers, imports, and fixtures that the current step legitimately touches.

When applying it, mention the cleanup in the commit message under a separate "Boy scout cleanup:" bullet so reviewers can distinguish the load-bearing change from the style sweep.

Quick Reference: Syntax Rules

Rule 1: Pipe Operator (|>) Over Nesting

Data flows left-to-right through transformations. Never use $ for nesting.

-- CORRECT: Left-to-right data flow
escapeHtml text = text
  |> replace "&" "&"
  |> replace "<" "&lt;"
  |> replace ">" "&gt;"

-- WRONG: Dollar-sign nesting
escapeHtml text = replace ">" "&gt;" $ replace "<" "&lt;" $ replace "&" "&amp;" text

Rule 2: Do-Blocks for ALL Intermediate Bindings

Use do with let for all intermediate bindings — even in pure (non-monadic) code. Never use let..in or where.

-- CORRECT: do-block for pure code
range lo hi = do
  let values = enumFromTo lo hi
  Array values

-- WRONG: let..in
range lo hi =
  let values = enumFromTo lo hi
  in Array values

-- WRONG: where clause
range lo hi = Array values
  where values = enumFromTo lo hi

Rule 3: Case Expressions Only for Pattern Matching

All pattern matching happens in case..of. Never pattern match in function definitions.

-- CORRECT: case expression on constructors
withDefault fallback result =
  case result of
    Ok value -> value
    Err _ -> fallback

-- WRONG: pattern matching in function definition
withDefault fallback (Ok value) = value
withDefault fallback (Err _) = fallback

Rule 4: If-Then-Else for Bool Conditionals

Use if..then..else for Bool conditions. Never case match on True/False.

-- CORRECT: if-then-else
validate input =
  if Text.isEmpty input
    then Err EmptyInput
    else Ok (ValidatedInput input)

-- WRONG: case matching on Bool
validate input =
  case Text.isEmpty input of
    True -> Err EmptyInput
    False -> Ok (ValidatedInput input)

Rule 5: Descriptive Type Parameters

Type parameters must be descriptive. Never use single letters.

-- CORRECT: descriptive names
map :: forall element result. (element -> result) -> Array element -> Array result
foldl :: forall element accumulator. (element -> accumulator -> accumulator) -> accumulator -> Array element -> accumulator

-- WRONG: single-letter parameters
map :: forall a b. (a -> b) -> Array a -> Array b

Rule 6: Qualified Import Convention

Import types unqualified, then import the module qualified.

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

-- CORRECT: GHC/base modules with full path
import Data.Vector qualified
import Control.Monad qualified

-- WRONG: Unqualified module import
import Array
import Data.Text

Rule 7: NeoHaskell Modules First

Always prefer nhcore modules over base/hackage equivalents.

Rule 8: String Interpolation with [fmt|...|]

Use the fmt quasi-quoter for all string construction. Interpolation uses #{expression} syntax.

-- CORRECT: fmt quasi-quoter with #{} interpolation
greet name = [fmt|Hello #{name}!|]
logRetry count = [fmt|Retrying command (attempt #{count})|]

-- WRONG: String concatenation
greet name = "Hello " <> name <> "!"

Rule 9: Result Over Either

NeoHaskell uses Result error value instead of Either. Constructors are Ok and Err.

-- CORRECT: Result type
validate :: Text -> Result ValidationError ValidatedInput

-- WRONG: Either type
validate :: Text -> Either ValidationError ValidatedInput

Rule 10: Task Over IO

NeoHaskell uses Task err val instead of IO.

-- CORRECT: Task with yield
readConfig :: Task ConfigError Config

-- WRONG: IO
readConfig :: IO Config

Rule 11: Task.yield, Not pure/return

-- CORRECT
Task.yield value

-- WRONG
pure value
return value

Test Conventions

module MyModuleSpec (spec) where

import Core
import Test

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

Test Rules

1. spec :: Spec Unit return type
2. \_ -> for unused test parameter in it blocks
3. |> pipes in assertions — value |> shouldBe expected
4. Tests are IMMUTABLE — once written, never modify test expectations

GHC Extensions in Use

Performance Annotations

-- Add INLINE to small, frequently-called functions in hot paths
yield :: value -> Task _ value
yield value = Task (Applicable.pure value)
{-# INLINE yield #-}

Note on Strict Fields: The Strict extension is enabled project-wide, so all record fields are strict by default. Do NOT add redundant ! bang patterns. Use {-# UNPACK #-} on primitive fields for unboxing.

Reference Files