Parameterized active patterns to the rescue! let (|Prefix|_|) (p:string) (s:string) = if s.StartsWith(p) then Some(s.Substring(p.Length)) else None match “Hello world” with | Prefix “The” rest -> printfn “Started with ‘The’, rest is %s” rest | Prefix “Hello” rest -> printfn “Started with ‘Hello’, rest is %s” rest | _ -> printfn “neither”
Having formerly been one of “those people”, I don’t know that there’s a succinct way to sum up why pattern-matching is such tasty goodness. It’s experiential. Back when I had just glanced at pattern-matching and thought it was a glorified switch statement, I think that I didn’t have experience programming with algebraic data types (tuples … Read more
Simply List.choose id as in > [Some 4; None; Some 2; None] |> List.choose id;; val it : int list = [4; 2] List.choose id
This is easy once you realize that Object-Oriented Constructor Injection corresponds very closely to Functional Partial Function Application. First, I’d write Dings as a record type: type Dings = { Lol : string; Rofl : string } In F#, the IGetStuff interface can be reduced to a single function with the signature Guid -> seq<Dings> … Read more
The inline keyword indicates that a function definition should be inserted inline into any code which uses it. Most of the time, this will not have any effect on the type of the function. However, in rare cases, it can lead to a function which has a more general type, since there are constraints which … Read more
What order are the files in the .fsproj file? Stack.fs needs to come before Program.fs for Program.fs to be able to ‘see’ it. See also the start of http://lorgonblog.spaces.live.com/blog/cns!701679AD17B6D310!444.entry and the end of http://lorgonblog.spaces.live.com/blog/cns!701679AD17B6D310!347.entry
I can only support what gradbot said – when I need mutation, I prefer let mutable. Regarding the implementation and differences between the two – ref cells are essentially implemented by a very simple record that contains a mutable record field. You could write them easily yourself: type ref<‘T> = // ‘ { mutable value … Read more
A namespace is a .Net thing, common in many industrial-strength languages, just a way to organize frameworks and avoid naming conflicts among different libraries. Both you and I can define a type “Foo” and use them both in a project, provided they are in different namespaces (e.g. NS1.Foo and NS2.Foo). Namespaces in .Net contain types. … Read more
The French and British descendants of the original ML made different choices and their choices have been inherited through the decades to the modern variants. So this is just legacy but it does affect idioms in these languages. Functions are not recursive by default in the French CAML family of languages (including OCaml). This choice … Read more
This question has been answered for some time now, but I was quite surprised that most of the answers say what OCaml features are missing in F# – this is definitely good to know if you want to port existing OCaml programs to F# (which is probably the motivation of most of the referenced articles). … Read more