F# - Active patterns

I like active patterns. I have yet to explore their possibilities completely but one pattern which crops up all the time in our code is checking whether a string is empty and acting accordingly.

In C# we have

if(!String.IsNullOrEmpty(myString))
{
    doSomethingWithTheString(myString);
}
else
{
    doSomethingInNullCase();
}

Clearly, the

doSomething...()

methods will usually be more than a simple procedure, often involving local variables and transformations thereof, but the essence of the pattern is captured above.

We could, of course, transliterate this directly into F# and that is the first step in the conversion process. Thus we have

if(not <| String.IsNullOrEmpty myString) then
    doSomethingWithTheString(myString)
else
    doSomethingInNullCase()

So what? I hear you say. Well, this is a step in the right direction. Functional programmers prefer pattern matching to indiscriminate use of the

if

statement though. The question, therefore, is what pattern can we match to achieve the end and how. Crucially, in F# one cannot match against a general function. Instead one uses what is known as an Active Pattern. There are three forms of Active Patterns: single-valued, partial and multi-valued. The names are mine but they are similar to the official names. I think of 'partial' active patterns as slightly different from the other two.

First, a single-valued active pattern:

let (|EmptyString|) str = String.IsNullOrEmpty str

which could be used in this manner:

match myString with | EmptyString true -> "Empty" | _ -> "Not empty"

I think the syntax for active patterns leaves a little to be desired so let me explain what is happening here. Clearly the match is being performed on

myString

. The match criterion first considered is

EmptyString

. All bar the final parameter (in this case, there are no additional parameters in the match criterion) are evaluated in the context of

myString

. The final parameter in the matching pattern is compared with the return value of the active pattern. In other words, the criterion is

EmptyString myString

which returns a

bool

against which we match

true

.

A multi-valued pattern is similar. For example:

let (|EmptyString|String|) str =
    if String.IsNullOrEmpty str then EmptyString else String(str)

Notice that this pattern looks much more like a discriminated union. Also, whilst it is not necessary to return a value in the 'constructor', if you will, in either case, we do so here for illustrative purposes. This pattern could be used as follows:

match String.Format("Is {0} empty?", myString) with
| EmptyString -> "It's empty"
| String str -> str

Thus

str

holds the string that was successfully matched.

Partial active patterns are a slight change again:

let (|NotEmpty|_|) s = if String.IsNullOrEmpty s then None else Some(s)

Note that I have changed the specified portion of the pattern to

NotEmpty

so that we can return the string that was matched successfully. Note also that this pattern returns an

option

type where the multi-valued pattern returns a

Choice

.

The partial pattern is used similarly:

match String.Format("Is {0} empty?", myString) with
| NotEmpty str -> str
| _ -> "It's empty"

As you can see, the construct is powerful and I haven't demonstrated how to pass additional parameters or match multiple values. Check Wikibooks for further information and then try the broader t'internet.

Reflection - the perils

I've been struggling with Reflection. I want to create an object from a dynamically loaded assembly. In general, this is a simple problem. Just use Type.InvokeMember, easy!

However, the problem comes when the constructor does other stuff, particularly when it loads yet more objects by reflection from dynamically loaded libraries. When it doesn't work, it can be really difficult to diagnose.

The upshot of this is that it would be so much better if the constuctor did the minimum amount and all the initialisation came later, once the object had been created etc.

Just a thought.

Creating a class with multiple constructors in F#

Using the implicit type syntax couldn't be easier. For example:


type A (a,b) as this =
let c = "sum"
do printfn "The sum is " (a+b)


Easy! But class A only has one constructor. Suppose we want another? The obvious guess would be


type B (a,b) as this =
let c = "sum"
do printfn "The sum is " (a+b)
new (d) = B(a,0)


but this doesn't work for a number of reasons. You can't have let or do sections in your type if you have multiple constructors. So this means that a, b and c have to be defined in the new function. This means, of course, that you need to use explicit class syntax. The action can then follow the initialisation in a then clause.


type C =
val a :float
val b :int
val c :string
new (_a,_b) = { a=_a; b=_b; c="sum"} then printfn "The sum is " (a+(float b))
new (_a) = { a=_a; b=0; (* Yes, you have to initialise all the class members *) c="sum"} then printfn "The sum is " (a+(float b))


Whew! Complicated! And repetitive, potentially! It is possible to call functions in the then clause but you still have to initialise all the members in the record clause.

It's more fiddly in the presence of inheritance unfortunately. In this case the record initialiser has to include inherit Cbase(a).

Here's one I've used in production code, anonymised...


type drv = class
inherit bse

val _parser : Parser
val _columnExtractors : List

new (configuration:ExpConfig ) as x =
{inherit bse(configuration); _parser = new Parser(configuration); _columnExtractors = new List()}
then x.Init()
new (configuration:ExpConfig, parser:Parser) as this =
{inherit bse(configuration); _parser = parser; _columnExtractors = new List()}
then this.Init()

member private x.Init() =
printfn "blah blah blah"
end


F# - Abstract class

It's possible to create an abstract class, but it's not obvious how to do it. You need to create a type, as normal. Then place an abstract method declaration within. At this point, though, the compiler still grumbles. Now you have to tag the type with [<AbstractClass>].

First attempt:


> type C =
inherit Exception
new () = {inherit Exception()}
abstract print : unit -> unit
;;

type C =
-----^

stdin(14,6): error FS0365: No implementation was given for 'abstract member C.print : unit -> unit'


Now apply the annotation and, hey presto, the compiler responds with:


type C =
class
inherit Exception
new : unit -> C
abstract member print : unit -> unit
end


But why the compiler can't work it out for itself I don't know.

Overloaded constructors and inheritance - F#

I've been struggling vainly to create my own Exception class but the obvious fails to compile:


type MyException1 () =
inherit Exception ()
new (msg:string) = {inherit Exception(msg)}


with the following error message:

      new (msg:string) = {inherit Exception(msg)}
  -----------------------^^^^^^^^^^^^^^^^^^^^^^^^

stdin(9,24): error FS0762: Constructors for the type 'MyException1' must directly or indirectly call its implicit object constructor. Use a call to the implicit object constructor instead of a record expression.

After some head-scratching, I guessed that the problem might be the default constructor, and lo, this works.


type MyException =
inherit Exception
new () = {inherit Exception()}
new (msg:string) = {inherit Exception(msg)}


It's not clear to me why I need to explicitly define the default constructor, but the compiler has stopped complaining now.