Named parameters, also known as keyword arguments, have several advantages over positional arguments:

convenience: they can be supplied in arbitrary order

readability: their names serve as documentation at call site

safety: it is impossible to accidentally mix them up

Consider a function to replace a substring with another string:

Text.replace path "$HOME" "/home/username/"

We want to replace references to the $HOME environment variable with a concrete directory. There is but one problem – we have supplied the text arguments in the wrong order.

Compare that to a newtype-based solution:

Text.replace (Needle "$HOME") (Replacement "/home/username/") (Haystack path)

Now that the function requires each argument to be wrapped in a newtype, we cannot mix them up – the compiler will report an error, and newtype constructors serve as documentation.

The problem with newtypes is that it is bothersome to create them for each parameter, they pollute the global namespace, and we still cannot supply wrapped arguments in arbitrary order.

With keyword arguments, none of that is a problem:

Functions must declare their parameter names in the type signature:

replace :: Text ` Named ` "needle" -> Text ` Named ` "replacement" -> Text ` Named ` "haystack" -> Text replace (Named needle) (Named replacement) (Named haystack) = ...

Keyword arguments have seamless interoperability with positional arguments when the function takes them last. Consider this function:

foo :: A -> B -> C ` Named ` "x" -> IO ()

There are several ways to invoke it:

(foo a b) ! #x c -- parentheses for clarity (foo a ! #x c) b -- parentheses required (foo ! #x c) a b -- parentheses required

We can also supply keyword arguments using the with combinator instead of the ! operator:

( with #x c foo) a b -- parentheses for clarity with #x c (foo a b) -- has the same effect

Both ! and with work in a similar manner: they traverse the spine of the function and supply the first keyword argument with a matching name.

For example: