F#：声明 function 并在列表中查找元素

Question

I'm quite new to F# and need some help with a problem. I'm having trouble finding an element in a list: "Declare a function findRoute: Lid*LuggageCatalogue -> Route, that finds the route for a given luggage identification in a luggage catalogue"

I have listed the different types and given the types elements.

    type Lid = string 
    type Flight = string
    type Airport = string
    type Route = (Flight*Airport) list
    let route = [("DL 189","ATL"); ("DL 124","BRU"); ("SN 733","CPH"); ("SK 208","ATL"); ("DL 124","BRU"); ("SK 122","JFK")]

    type LuggageCatalogue = (Lid*Route) list
    let lc = [("DL 016-914", [("DL 189","ATL"); ("DL 124","BRU"); ("SN 733","CPH")]); 
    ("SK 222-142", [("SK 208","ATL"); ("DL 124","BRU"); ("SK 122","JFK")])]


    let findRoute fr = function
    match fr with
    | [] -> printf "No match"
    |

Where I'm stuck is that I cannot find the correct way to use the function to find the elements in the LuggageCatalogue list

Answer 1

Maybe you could start by filtering the luggage catalogue to better understand what is going on, eg

let filterlist lid = List.filter (fun (luggageid, _) -> luggageid = lid)

Then finding a particular item in the catalogue would be something like

filterlist "SK 222-142" lc

Or even better, try to create the first part of the function yourself. Even though this is only the first step, maybe try to visualize how a recursive search function for a luggage id may look,

let findfirst lid =
  let rec checktail lctail =
    match lctail with
    | [] -> invalidArg "lid" "luggageid not present in catalogue"
    | (id, r)::tail -> if id = lid then r else checktail tail
  checktail lc