Lines starting with -- are ignored by the compiler, and are useful for describing something to human readers.

-- Nothing to see here

You can also use {- -} to have comments span multiple lines.

{- This comment
multiple lines

Comments can be used pretty much everywhere, as they're completely ignored by the compiler.

Documentation comments

There is a special kind of comments, called documentation comments, which are recognized by the compiler as being documentation for a module or a value.

These comments are enclosed within {-| -} and will be used to render documentation on

{-| This is an approximation of pi
pi = 3.14