Optional Parameters in Haskell

I use optional parameters in my TagSoup library, but it seems not to be a commonly known trick, as someone recently asked if the relevant line was a syntax error. So, here is how to pass optional parameters to a Haskell function.

Optional Parameters in Other Languages

Optional parameters are in a number of other languages, and come in a variety of flavours. Ada and Visual Basic both provide named and positional optional parameters. For example, given the definition:

Sub Foo(b as Boolean = True, i as Integer = 0, s as String = "Hello")

We can make the calls:

Call Foo(s = "Goodbye", b = False)
Call Foo(False, 1)

In the first case we give named parameters, in the second we give all the parameters up to a certain position.

In some languages, such as GP⁺, you can say which parameters should take their default values:

Call Foo(_, 42, _)

Optional Parameters in Haskell

Haskell doesn't have built-in optional parameters, but using the record syntax, it is simple to encode named optional parameters.

data Foo = Foo {b :: Bool, i :: Integer, s :: String}
defFoo = Foo True 0 "Hello"

foo :: Foo -> IO ()

Now we can pass arguments by name, for example:

foo defFoo{s = "Goodbye", b = False}

This syntax takes the value defFoo, and replaces the fields s and b with the associated values. Using a type class, we can abstract this slightly:

class Def a where
    def :: a

instance Def Foo where
    def = defFoo

Now we can make all functions taking default arguments use the def argument as a basis, allowing type inference and type classes to choose the correct default type. Even still, optional parameters in Haskell are not quite as neat as in other languages, but the other features of Haskell mean they are required less often.

This technique has been used in TagSoup, particularly for the parseTagOptions function. I've also seen this technique used in cpphs with the runCpphs function.