Wednesday, December 12, 2012

Neat F#: Custom Operators

F# has support for custom operators.  The best use of this I've seen so far is in the canopy web testing library.  Canopy allows you to write code like:
"#firstName" << "Kevin"
"#firstName" == "Kevin"
That code is the same as this code written with the Coypu web testing library:
browser.FillIn("#firstName").With("Kevin")
browser.FindField("#firstName").Value.ShouldEqual("Kevin")
As you've likely deduced, the "<<" operator has been overridden to lookup the field and set it's value, while the "==" operator has been overridden to lookup up the field and assert on it's value.

In this case, both of these operators do exist already in F#, but they obviously aren't usually used to drive a web browser.  So this is a powerful use of operator overloading.  But F# allows you to define custom operators that have no definition in F# as well.  They can be any combination of a certain set of characters.

For example, there is no "=~" operator in F#, but you could define one to do a regex match as follows:
open System.Text.RegularExpressions
let (=~) input pattern = Regex.IsMatch(input, pattern)
And you'd use it like:
"some input" =~ ".*input"
And you could also define one that is case insensitive:
let (=~*) input pattern = Regex.IsMatch(input, pattern, RegexOptions.IgnoreCase)
These operators are not overloaded, they're just custom defined.

There is clearly a tradeoff here between explicit code and concise code.  Look back at the first example from Canopy again.  If you knew that was web testing code, and you recognized "#firstName" as a css selector, you would probably figure out what it was doing.  And this conciseness is going to be really nice in a situation where you're executing the same type of operations over and over and over again (say, like, in a Selenium web test!).  So while there's no mistaking what the Coypu code is doing, I'd rather write the Canopy code!

However, in the regular expression example, since =~ and =~* are not part of the language, how would you know what they do.  Certainly there's a similarity to ruby, but I've never seen a =~* operator.  So introducing stuff like this to your code base runs the risk of making your code harder to understand.

In the end, I think it's an awesome feature to have at your disposal.  And I think a good rule of thumb is to be willing to try some custom operators when you have a high and dense repetition of operations.  That is, it's not a one off operation, or it's not used always by itself in far flung sections of code.

In any case, this another powerful, and very neat, feature of F#.