Back to Questions
913
views1
comment
The 'a type
f#

Hi,

As a beginner F# learner, I found the most difficult part in learning the language is to understand its type system. For example, given this code snippet from Don’s book:

let getValue a =

match a with

| (("lo" | "low") ,v) -> v

| ("hi",v) | ("high",v) -> v

| _ -> failwith "expected a both a high and low value";;

It took me a while to get used to the concept that this code could even work without “v” being declared on the “let” line. Finally, I started to think that the ‘a type, i.e. the “any” type, is probably equivalent to the <T> in C++/Java. So if I were to rewrite the let binding from the above code in C++/Java, I could have written it as:

template class< T> T getValue(PairClassType a, T v); // C++

<T> getValue(PairClassType a, T v); // Java

I know the C++/Java code is not the same as the F# code but hopeful I got my point across, i.e. the F# code automatically gives me a generalized type on “v” without requiring a superficial type declaration of <T>. The build-in tuple type in F# also saves a type definition of “PairClassType” in C++/Java. But is this understanding correct?

Thanks.

da

.

You could think of this code as being similiar to this Java code.

1
2
3
4
5
6
7

<T> T getValue(Pair<String,T> a) {
  String s = a.getFst();
  T v = a.getSnd();
  if(s.Equals("lo") || s.Equals("low")) return v;
  else if(s.Equals("hi") || s.Equals("high")) return v;
  else throw new Exception("expected a both a high and low value");
}

Patterns in match blocks can bind new variables. This is how the "v" is declared.

By on 5/8/2008 5:30 AM ()

    Topic tags

    IntelliFactory Offices Copyright (c) 2011-2012 IntelliFactory. All rights reserved.
    Home | Products | Consulting | Trainings | Blogs | Jobs | Contact Us | Terms of Use | Privacy Policy | Cookie Policy     		Built with WebSharper