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Related Posts

• Part 1
• Part 2
• Part 3

 

Here I will look at how some com­monly used func­tions in F#’s List mod­ule might be trans­lated to Erlang using Erlang’s equiv­a­lent – the lists module.

 

List.append

F#:         let newList = List.append [ 1..5 ] [ 6..10 ]

Erlang:   NewList = lists:append([ 1, 2, 3, 4, 5 ], [ 6, 7, 8, 9, 10 ]).

List.average

F#:         let avg = List.average [ 1.0..10.0 ]

Erlang:   List = lists: seq(1, 10).

             Avg = lists: sum(List) / length(List).

List.collect

F#:         let duped = [ ‘a’; ‘b’; ‘c’ ] |> List.collect (fun x -> [ x; x ])

Erlang:   Duped = lists:flatmap(fun(X) –> [ X, X ] end, [ a, b, c ]).

List.concat

F#:         let newList = List.concat [ [ 1; 2 ]; [ 3; 4; 5 ]; [ 6; 7; 8; 9 ] ]

Erlang:   NewList = lists:append([ [ 1, 2 ], [ 3, 4, 5 ], [ 6, 7, 8, 9 ] ]).

 

List.exists

F#:         let hasEven = [ 1..2..9 ] |> List.exists (fun x -> x % 2 = 0)

Erlang:   HasEven = lists:any(fun(X) -> X rem 2 =:= 0 end, lists: seq(1, 9, 2)).

 

List.filter

F#:         let evens = [ 1.. 6 ] |> List.filter (fun x -> x % 2 = 0)

Erlang:   Evens = lists:filter(fun(X) -> X rem 2 =:= 0 end, lists:seq(1, 6)).

 

List.find

F#:         let fst­Match = [ “cui”; “xue”; “yan” ] |> List.find ((=) “yan”)

Erlang:   Fst­Match = lists:keyfind(yan, 1, [ { cui }, { xue }, { yan } ]).

 

List.fold

F#:         let data = [ (“Cats”, 4); (“Dogs”, 5); (“Mice”, 3); (“Ele­phants”, 2) ]

              let count = data |> List.fold (fun acc (_, x) -> acc + x) 0

Erlang:   Data = [ { cats, 4 }, { dogs, 5 }, { mice, 3 }, { ele­phants, 2 } ].

             Count = lists:foldl(fun({ _, N }, Acc) -> Acc + N end, 0, Data).

 

List.foldBack

F#:         let copy = List.foldBack (fun elem acc -> elem::acc) [ 1..10 ] []

Erlang:   Copy = lists:foldr(fun(X, Acc) -> [X|Acc] end, [], lists:seq(1, 10)).

 

List.forall

F#:         let allEven = [2; 4; 6; 8] |> List.forall (fun n -> n % 2 = 0)

Erlang:   AllEven = lists:all(fun(X) -> X rem 2 =:= 0 end, [ 2, 4, 6, 8 ]).

 

List.iter

F#:         [1..10] |> List.iter (printfn “%d”)

Erlang:   lists:foreach(fun(X) -> io:format(“~p~n”, [ X ]) end, lists:seq(1, 10)).

 

List.length

F#:         let lst = [ 1..10 ]

              // you can call the Length prop­erty on a list

              let len1 = lst.Length

              // or use List.length

              let len2 = List.length lst

Erlang:   Lst = lists: seq(1, 10).

             % you can use the length built-in function

             Len1 = length(Lst).

             % or you can use lists:flatlength

             Len2 = lists:flatlength(Lst).

 

List.map

F#:         let dou­bles = [ 1..10 ] |> List.map (fun x -> x * 2)

Erlang:   Dou­bles = lists:map(fun(X) -> X * 2 end, lists:seq(1, 10)).

 

List.max

F#:         let max = [ 1..10 ] |> List.max

Erlang:   Max = lists:max(lists: seq(1, 10)).

 

List.min

F#:         let min = [ 1..10 ] |> List.min

Erlang:   Min = lists:min(lists: seq(1, 10)).

 

List.nth

F#:         let fourth = List.nth [ 1..10 ] 3

Erlang:   Fourth = lists:nth(4, lists: seq(1, 10)). % node that n is not zero-indexed here!

 

List.partition

F#:         let evens, odds = [ 1..10 ] |> List.partition (fun n -> n % 2 = 0)

Erlang:   { Evens, Odds } = lists:partition(fun(X) -> X rem 2 =:= 0 end, lists:seq(1, 10)).

 

List.rev

F#:         let rev = [ 1..10 ] |> List.rev

Erlang:   Rev = lists:reverse(lists: seq(1, 10)).

 

List.sort

F#:         let sorted = List.sort [ 1; 4; 7; –1; 5 ]

Erlang:   Sorted = lists:sort([ 1, 4, 7, –1, 5 ]).

 

List.sum

F#:         let sum = List.sum [ 1..10 ]

Erlang:   Sum = lists:sum(lists:seq(1, 10)).

 

List.unzip

F#:         let lstA, lstB = List.unzip [ (1, 2); (3, 4) ]

Erlang:   { LstA, LstB } = lists:unzip([ {1, 2}, {3, 4} ]).

 

List.zip

F#:         let lst = List.zip [ 1; 2 ] [ 3; 4 ]

Erlang:   Lst = lists:zip([ 1, 2 ], [ 3, 4 ]).

Related Posts

• Part 1
• Part 2
• Part 4

 

Throw­ing Exceptions

In F#, you can define a cus­tom excep­tion type by cre­at­ing a type that inherit from System.Exception or using the light­weight excep­tion syn­tax to define them with the same syn­tax as dis­crim­i­nated unions.

You can throw excep­tions in F# using the raise key­word or using fail­with or fail­withf:

excep­tion MyExceptionA

excep­tion MyEx­cep­tionB of string

let exThrower (isA : bool option) =

    match isA with

    | Some(true) -> raise MyExceptionA

    | Some(false) -> raise <| MyExceptionB(“Blah Blah”)

    | _ -> raise <| new System.Exception()

    | _ -> fail­with “Not a valid input”

Erlang also has a num­ber of dif­fer­ent excep­tion types, the most com­mon ways to raise an excep­tion in Erlang is via:

• error(Reason) – ter­mi­nates the cur­rent process and includes a stack trace.
• exit(Reason) – sim­i­lar to errors, but does not return the stack trace.
• throw(Reason) – throws excep­tions that the caller can be expected to han­dle, also used as a way to return from deep recursion.

1> throw(“Blah Blah”).

** excep­tion throw: “Blah Blah”

2> error(boo).

** excep­tion error: boo

3> exit(foo).

** excep­tion exit: foo

 

Catch­ing Exceptions

In F#, you can imple­ment a try-catch block using the try-with key­words, and it’s really easy to use pat­tern match­ing to make han­dling mul­ti­ple types of excep­tions really easy.

Bor­row­ing from the won­der­fully humor­ous exam­ple from Learn you some Erlang for great good! here’s how it’ll look in F#:

 

In Erlang, the syn­tax is sur­pris­ingly similar:

 

Another thing to keep in mind is that whilst F# has try-with and try-finally blocks there is no equiv­a­lent of C#’s try-catch–finally block. In Erlang the try-catch-finally block looks like this:

black_knight(Attack) when is_function(Attack, 0) –>

    try Attack() of

        _ -> “None shall pass.”

    catch

        throw:slice   -> “It is but a scratch.”;

        error:cut_arm -> “I’ve had worse.”;

        exit: cut_leg -> “Come on you pansy!”;

        _:_             -> “Just a flesh wound.”

    after

        io:format(“Attack finished…~n”, [])

    end.