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I see Haskell multi-line comments that sometimes start with {- | instead of just {-.
Does leading with the pipe character inside the comment mean something by convention?
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The “-- |” syntax begins a documentation annotation, which applies to the following declaration in the source file. Note that the annotation is just a comment in Haskell — it will be ignored by the Haskell compiler.
It's merely an infix synonym for fmap , so you can write e.g. Prelude> (*2) <$> [1.. 3] [2,4,6] Prelude> show <$> Just 11 Just "11" Like most infix functions, it is not built-in syntax, just a function definition. But functors are such a fundamental tool that <$> is found pretty much everywhere.
a -> b Bool means... forall (a :: *) (b :: * -> *). a -> b Bool. b is therefore a type constructor taking a single type argument. Examples of single-argument type constructors abound: Maybe , [] , IO are all examples of things which you could use to instantiate b .
The @ Symbol is used to both give a name to a parameter and match that parameter against a pattern that follows the @ . It's not specific to lists and can also be used with other data structures.
The | at the start of a comment is Haddock syntax that begins a documentation annotation. An example from the Haddock documentation is:
-- |The 'square' function squares an integer. square :: Int -> Int square x = x * x It also goes on to say
The “-- |” syntax begins a documentation annotation, which applies to the following declaration in the source file. Note that the annotation is just a comment in Haskell — it will be ignored by the Haskell compiler.
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