F# Iterate through sequence and call a function for each element of sequence

Question

I need to call a function for each element of a sequence, currently I have tried Seq.iter and Seq.map but they return unit and 'a ->'c respectively and not Json like I need.

I have tried

Seq.iter (fun _ (a,b,c,d) -> iterateThroughMySequnce a b c d ()) sequence
Seq.fold (fun _ (a,b,c,d) -> iterateThroughMySequnce a b c d ()) sequence

but i am not getting the expected return type of Json. The code is needed below where the comment says "Something like this"

Can anyone help Thanks

open Newtonsoft.Json.Linq

type Json =
    | JObj of Json seq
    | JProp of string * Json
    | JArr of Json seq
    | JVal of obj

let (!!) (o: obj) = JVal o

let rec toJson = function
    | JVal v -> new JValue(v) :> JToken
    | JProp(name, (JProp(_) as v)) -> new JProperty(name, new JObject(toJson v)) :> JToken
    | JProp(name, v) -> new JProperty(name, toJson v) :> JToken
    | JArr items -> new JArray(items |> Seq.map toJson) :> JToken
    | JObj props -> new JObject(props |> Seq.map toJson) :> JToken

let sequence = seq { yield "USD", 12.36M, 156.32M, 18.23M
                     yield "JPY", 13.36M, 564.32M, 17.23M 
                     yield "GBP", 14.36M, 516.32M, 120.23M }

let iterateThroughMySequnce a b c d () =
    JObj [JProp("CurrencyCode", !! a);
          JProp("TotalPerCurrencyBeforeExchange", !! b); 
          JProp("ExchangeRate", !! c);
          JProp("TotalPerCurrencyAfterExchange", !! d)];

let k =
    JObj [
        JProp("InvoiceNumber", !! "13456789");
        JProp("InvoiceDate", !! "21/12/2015");
        JProp("InvoiceCurrency", !! "USD");
        JProp("InvoiceProfitMargin", !! 2.3);
        JProp("InvoicePaymentCurrencyToEuroExchangeRate", !! 0.8658745M);
        JProp("InvoicePeroid", !! "01/01/2015 00:00:00 - 01/02/2015 23:59:59");
        JProp(
            "Transaction", 
                JArr [
                    //Something like this
                    Seq.iter (fun (a,b,c,d) -> iterateThroughMySequnce a b c d ()) sequence
                ])
        JProp("TransactionForPeroid", !! 254584.00M);
        JProp("InvoicingAmountWithProfitMarginApplied", !! 8452.01M);
        JProp("InvoicingAmountWithProfitMarginAppliedInEuro", !! 7851.28);
    ]

let json = toJson k 

Answer 1

You need Seq.map, which transforms an input sequence into an output sequence (and turns each element into a new value using the specified function). Your code is almost right, but the call should not be wrapped in another list:

JProp(
    "Transaction", 
        JArr (Seq.map (fun (a,b,c,d) -> iterateThroughMySequnce a b c d ()) sequence)
)

You can make this nicer if you change your iterateThroughMySequence function to accept a tuple (and also, it should be named differently, because it is not iterating!)

let formatItemAsJson (a,b,c,d) =
  JObj [JProp("CurrencyCode", !! a);
        JProp("TotalPerCurrencyBeforeExchange", !! b); 
        JProp("ExchangeRate", !! c);
        JProp("TotalPerCurrencyAfterExchange", !! d)];

// Later in the main part of code
JProp("Transaction", JArr (Seq.map iterateThroughMySequnce sequence))

Aside, the F# Data library comes with JsonValue type (see the API reference), which implements some of what you're doing here - it lets you construct & format JSON (and also parse it).