Welcome to the eighth article in my series about why I look upon the
the hormone-driven lust of a 16-year old boy. ([ed.] Dustin's trophy wife has indicated
that the previous metaphor might be a little too
If you're just joining us, below is the path that has brought us to this
Type-safe Format Strings
Breaking Up Tuples
Functions, Functions, Functions!
- Pattern Matching
Today, we're taking a high-level look at F# option types. Option
types are a simple example of a discriminated (or tagged)
union1, although understanding that isn't necessary in order to use them. Simply put, an option type wraps
a value with information indicating whether or not the value exists. For C# or VB programmers, it
may be convenient to think of option types
as a mutant cross between
.NET 2.0 nullable types and the
null object design
There are two constructors that instantiate option types. First,
there's the Some
constructor, which takes a value to be wrapped.
> let someValue = Some(42);;
val someValue : int option
And then, there's the None constructor,
which doesn't take anything.
> let noValue = None;;
val noValue : 'a option
Notice that, in the above
code, F# infers the type of noValue as the generic, 'a option, rather than int option.
That's because, unlike the declaration of someValue,
information indicates an int. If you really want to declare a
as type int option, you'd declare it like so:
> let noValue : int option = None;;
val noValue : int option
One of the properties of option types that makes them so compelling is the
ability to pattern match over them.
> let isFortyTwo opt =
- match opt with
- | Some(42) -> true
- | Some(_) -> false
- | None -> false;;
val isFortyTwo : int option -> bool
Now, we can call our isFortyTwo
function to show that the pattern matching works as expected.
> isFortyTwo someValue;;
val it : bool = true
> isFortyTwo noValue;;
val it : bool = false
> isFortyTwo (Some(41));;
val it : bool = false
This is all well and good, but we need a practical example to sink our teeth
into. Let's use the .NET Framework
Stream.ReadByte function as a guinea pig. ([ed.] Dustin is not
implying that you should sink your teeth into guinea pigs. That's disgusting.
Shame on you.)
Stream.ReadByte has a pretty bad code smell. First of all, it returns
an int instead of a byte. Initially, that should seem strange since the method
specifically states that it's a byte generator.
returns -1 when
the current position is at the end of the stream. Because -1 is not expressible
as an unsigned byte, ReadByte returns an int. Of course, that's the second problem:
information is encoded into the result value of this function. However, unless you read the documentation, there's no way of knowing
By employing an option type, we can clarify the function and be a bit more honest about
> open System.IO
- let readByte (s : #Stream) =
- match s.ReadByte() with
- | i when i < 0 -> None
- | i -> Some(Byte.of_int i);;
val readByte : #System.IO.Stream -> byte option
Now, the semantics of the function are better expressed thanks to the option
In addition, we can write a function that pattern matches over the result of our readByte function.
> let rec printStream s =
- match readByte s with
- | Some(b) ->
- printfn "%d" (Byte.to_int b)
- printStream s
- | _ -> ();;
val printStream : #Stream -> unit
And here's the above printStream
function in action:
> let bytes = [|1uy .. 10uy|];;
val bytes : byte array
> let memStream = new MemoryStream(bytes);;
val memStream : MemoryStream
> printStream memStream;;
val it : unit = ()
Option types provide an elegant way to attach a bit of extra boolean information
to a value. It's important to become comfortable with them as they are
used extensively throughout the F# libraries.
Have fun! Next we'll explore... well... I haven't decided yet. If you have
any suggestions, feel free to email me at dustin AT diditwith.net.
1We'll explore discriminated unions in a future article.