Comments and Style

Comments

-- A single-line comment

{- A block comment
   spanning several lines.
   {- Block comments can nest. -} -}

-- | Returns the price including tax.
--
-- >>> withTax 100
-- 120.0
withTax :: Double -> Double
withTax price = price * 1.2 -- after code

main :: IO ()
main = print (withTax 100)

120.0

Comments starting with -- | are Haddock documentation comments: the haddock tool (and cabal haddock) turns them into HTML docs, and your editor shows them on hover. The >>> lines are examples that the doctest tool can run as tests.

Style guidelines

Common conventions in the Haskell community:

• A type signature on every top-level definition.
• camelCase for functions and values, PascalCase for types, constructors, type classes, and modules.
• Two-space (or four-space) indentation; spaces, never tabs (GHC warns about tabs).
• Short names for very local, generic values (x, xs, f, acc), descriptive names for everything else. The s suffix means "plural": (x : xs).
• Small functions that do one thing, composed together.
• Pattern matching and Maybe instead of head, tail, fromJust, and !!.
• Compile with -Wall and fix the warnings.

Tools

• Ormolu / Fourmolu — format code automatically.
• HLint — suggests simplifications, for example concat (map f xs) → concatMap f xs.
• Haskell Language Server — types on hover, errors as you type, and one-click HLint fixes.

What's next

You now know the core of Haskell: types, functions, lists, pattern matching, and recursion. Try the Beginner Projects, then continue to Level 2 — Intermediate, which starts with higher-order functions and goes on to your own data types, type classes, and monads.