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Adventures in F# - Part 4

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Moving on in the TryFSharp.org site today I finished the last two sections in the Advanced part.

Notes

  • Computational Expressions can be used to “alter the standard evaluation rule of the language and define sub-languages that model certain programming patterns”

    The reason we can alter the semantics of the language is the F# compiler rewrites (de-sugars) computation expressions before compiling.

    For example the code snippet below

      type Age =
  | PossiblyAlive of int
  | NotAlive
	
  type AgeBuilder() =
      member this.Bind(x, f) =
          match x with
          | PossiblyAlive(x) when x >= 0 && x <= 120 -> f(x)
          | _ -> NotAlive
      member this.Delay(f) = f()
      member this.Return(x) = PossiblyAlive x
	
  let age = new AgeBuilder()
	
  let willBeThere a y =
    age { 
      let! current = PossiblyAlive a
      let! future = PossiblyAlive (y + a)
	
      return future
    }
  willBeThere 38 150

    is de-sugared to this:

      let willBeThere2 a y =
    age.Delay(fun () -> 
      age.Bind(PossiblyAlive a, fun current ->
        age.Bind(PossiblyAlive (y+a), fun future ->
          age.Return(future))))
	
  willBeThere2 38 80

    At this point this concept seems too complicated to sink my teeth into. TryFSharp.suggests further study of the following concepts to have a better understanding of computation expressions:

    • The sophisticated architecture of function calls that are generated by de-sugaring
    • Monads - the theory behind computational expressions

  • Quotations is a language feature “that enables you to generate and work with F# code expressions programmatically. This feature lets you generate an abstract syntax tree that represents F# code. The abstract syntax tree can then be traversed and processed according to the needs of your application. For example, you can use the tree to generate F# code or generate code in some other language.”

Conclusion

After dabbling 4 days in TryFSharp.org I think it’s time to move on. There are 4 more sections in the site but since I can only learn by building something on my own I’ll try to come up with a small project to use the basics. Otherwise it’s likely that all this information will be forgotten. It’s already very overwhelming and I need a small achievement to motivate myself.

So I leave TryFSharp.org at this point, for a while at least. Even though I don’t find the advanced topics well-explained it’s still a very nice resource to get started.

TryFSharp.org

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