Part of the "Understanding F# types" series (more)

Now let's look at a particular union type, the Option type. It is so common and so useful that it is actually built into the language.

You have already seen the option type discussed in passing, but let's go back to basics and understand how it fits into the type system.

A very common situation is when you want to represent missing or invalid values. Using a diagram, the domain would look like this:

int option

Obviously this calls for some kind of union type!

In F#, it is called the Option type, and is defined as union type with two cases: Some and None. A similar type is common in functional languages: OCaml and Scala also call it Option, while Haskell calls it Maybe.

Here is a definition:

type Option<'a> =       // use a generic definition  
   | Some of 'a           // valid value
   | None                 // missing
IMPORTANT: if you evaluate this in the interactive window, be sure to reset the session afterwards, so that the built-in type is restored.

The option type is used in the same way as any union type in construction, by specifying one of the two cases, the Some case or the None case:

let validInt = Some 1
let invalidInt = None

and when pattern matching, as with any union type, you must always match all the cases:

match validInt with 
| Some x -> printfn "the valid value is %A" x
| None -> printfn "the value is None" 

When defining a type that references the Option type, you must specify the generic type to use. You can do this in an explicit way, with angle brackets, or use the built-in "option" keyword which comes after the type. The following examples are identical:

type SearchResult1 = Option<string>  // Explicit C#-style generics 
type SearchResult2 = string option   // built-in postfix keyword

Using the Option type

The option type is widely used in the F# libraries for values that might be missing or otherwise invalid.

For example, the List.tryFind function returns an option, with the None case used indicate that nothing matches the search predicate.

[1;2;3;4]  |> List.tryFind (fun x-> x = 3)  // Some 3
[1;2;3;4]  |> List.tryFind (fun x-> x = 10) // None

Let's revisit the same example we used for tuples and records, and see how options might be used instead:

// the tuple version of TryParse
let tryParseTuple intStr = 
   try
      let i = System.Int32.Parse intStr
      (true,i)
   with _ -> (false,0)  // any exception

// for the record version, create a type to hold the return result
type TryParseResult = {success:bool; value:int} 

// the record version of TryParse
let tryParseRecord intStr = 
   try
      let i = System.Int32.Parse intStr
      {success=true;value=i}
   with _ -> {success=false;value=0}  

// the option version of TryParse
let tryParseOption intStr = 
   try
      let i = System.Int32.Parse intStr
      Some i
   with _ -> None

//test it
tryParseTuple "99"
tryParseRecord "99"
tryParseOption "99"
tryParseTuple "abc"
tryParseRecord "abc"
tryParseOption "abc"

Of these three approaches, the "option" version is generally preferred; no new types need to be defined and for simple cases, the meaning of None is obvious from the context.

Option equality

Like other union types, option types have an automatically defined equality operation

let o1 = Some 42
let o2 = Some 42

let areEqual = (o1=o2)

Option representation

Option types have a nice default string representation, and unlike other union types, the ToString() representation is also nice.

let o = Some 42
printfn "%A" o   // nice
printfn "%O" o   // nice

Options are not just for primitive types

The F# option is a true first class type (it's just a normal union type, after all). You can use it with any type. For example, you can have an option of a complex type like Person, or a tuple type like int*int, or a function type like int->bool, or even an option of an option type.

type OptionalString = string option 
type OptionalPerson = Person option       // optional complex type
type OptionalTuple = (int*int) option       
type OptionalFunc = (int -> bool) option  // optional function
type NestedOptionalString = OptionalString option //nested options!
type StrangeOption = string option option option

How the Option type should not be used

The option type has functions such as IsSome, IsNone and Value, which allow you to access the "wrapped" value without doing pattern matching. Don't use them! Not only it is not idiomatic, but it is dangerous and can cause exceptions.

Here is how not to do it:

let x = Some 99

// testing using IsSome
if x.IsSome then printfn "x is %i" x.Value   // ugly!!

// no matching at all
printfn "x is %i" x.Value   // ugly and dangerous!!

Here is how to do it properly:

let x = Some 99
match x with 
| Some i -> printfn "x is %i" i
| None -> () // what to do here?

The pattern matching approach also forces you to think about and document what happens in the None case, which you might easily overlook when using IsSome.

The Option module

If you are doing a lot of pattern matching on options, look into the Option module, as it has some useful helper functions like map, iter and so on.

For example, say that I want to multiply the value of an option by 2 if it is valid. Here's the pattern matching way:

let x = Some 99
let result = match x with 
| Some i -> Some(i * 2)
| None -> None

And here's a more compact version written using Option.map:

let x = Some 99
x |> Option.map (fun v -> v * 2)

Or perhaps I want to multiply the value of an option by 2 if it is valid but return 0 if it is None. Here's the pattern matching way:

let x = Some 99
let result = match x with 
| Some i -> i * 2
| None -> 0

And here's the same thing as a one-liner using Option.fold:

let x = Some 99
x |> Option.fold (fun _ v -> v * 2) 0 

In simple cases like the one above, the defaultArg function can be used as well.

let x = Some 99
defaultArg x 0 

Option vs. Null vs. Nullable

The option type often causes confusion to people who are using to dealing with nulls and nullables in C# and other languages. This section will try to clarify the differences.

Type safety of Option vs. null

In a language like C# or Java, "null" means a reference or pointer to an object that doesn't exist. The "null" has exactly the same type as the object, so you can't tell from the type system that you have a null.

For example, in the C# code below we create two string variables, one with a valid string and one with a null string.

string s1 = "abc";
var len1 = s1.Length;

string s2 = null;
var len2 = s2.Length;

This compiles perfectly, of course. The compiler cannot tell the difference between the two variables. The null is exactly the same type as the valid string, so all the System.String methods and properties can be used on it, including the Length property.

Now, we know that this code will fail by just looking at it, but the compiler can't help us. Instead, as we all know, you have to tediously test for nulls constantly.

Now let's look at the nearest F# equivalent of the C# example above. In F#, to indicate missing data, you would use an option type and set it to None. (In this artificial example we have to use an ugly explicitly typed None -- normally this would not be necessary.)

let s1 = "abc"
var len1 = s1.Length

// create a string option with value None
let s2 = Option<string>.None
let len2 = s2.Length

In the F# version, we get a compile-time error immediately. The None is not a string, it's a different type altogether, so you can't call Length on it directly. And to be clear, Some [string] is also not the same type as string, so you can't call Length on it either!

So if Option<string> is not a string, but you want to do something with the string it (might) contain, you are forced to have to pattern match on it (assuming you don't do bad things as described earlier).

let s2 = Option<string>.None

//which one is it?
let len2 = match s2 with
| Some s -> s.Length
| None -> 0

You always have to pattern match, because given a value of type Option<string>, you can't tell whether it is Some or None.

In just the same way Option<int> is not the same type as int, Option<bool> is not the same type as bool, and so on.

To summarize the critical points:

  • The type "string option" is not at all the same type as "string". You cannot cast from string option to string; they do not have the same properties; and a function that works with string will not work with string option, and vice versa. So the type system will prevent any errors.
  • On the other hand, a "null string" in C# is exactly the same type as "string". You cannot tell them apart at compile time, only at run time. A "null string" appears to have all the same properties and functions as a valid string, except that your code will blow up when you try to use it!

Nulls vs. missing data

A "null" as used in C# is completely different from the concept of "missing" data, which is a valid part of modeling any system in any language.

In a true functional language there can be a concept of missing data, but there can be no such thing as "null", because the concepts of "pointers" or "uninitialized variables" do not exist in the functional way of thinking.

For example, consider a value bound to the result of an expression like this:

let x = "hello world"

How can that value ever be uninitialized, or become null, or even become any other value at all?

Unfortunately, additional confusion has been caused because in some cases API designers have used null to indicate the concept of "missing" data as well! For example, the .NET library method StreamReader.ReadLine returns null to indicate that there is no more data in a file.

F# and null

F# is not a pure functional language, and has to interact with the .NET languages that do have the concept of null. Therefore, F# does include a null keyword in its design, but makes it hard to use and treats it as an abnormal value.

As a general rule, nulls are never created in "pure" F#, but only by interacting with the .NET libraries or other external systems.

Here are some examples:

// pure F# type is not allowed to be null (in general)
type Person = {first:string; last:string}  
let p : Person = null                      // error! 

// type defined in CLR, so is allowed to be null
let s : string = null                      // no error! 
let line = streamReader.ReadLine()         // no error if null 

In these cases, it is good practice to immediately check for nulls and convert them into an option type!

// streamReader example
let line = match streamReader.ReadLine()  with
           | null -> None
           | line -> Some line

// environment example
let GetEnvVar var = 
    match System.Environment.GetEnvironmentVariable(var) with
    | null -> None
    | value -> Some value

// try it
GetEnvVar "PATH"
GetEnvVar "TEST"

And on occasion, you may need to pass a null to an external library. You can do this using the null keyword as well.

Option vs. Nullable

In addition to null, C# has the concept of a Nullable type, such as Nullable<int>, which seems similar to the option type. So what's the difference?

The basic idea is the same, but Nullable is much weaker. It only works on value types such as Int and DateTime, not on reference types such as strings or classes or functions. You canít nest Nullables, and they donít have much special behavior.

On the other hand, the F# option is a true first class type and can be used consistently across all types in the same way. (See the examples above in the "Options are not just for primitive types" section.)

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