CODEWITHSUNDEEP
listtypesaliaselm
Henk de Marie
Henk de Marie

Reputation: 77

how to effectively convert a list to type alias in Elm

I have this type alias:

type alias ResponseLine =
     { tag : Maybe String
    , vr : Maybe String
    , value : Maybe String
    , comments : Maybe String
    , len : Maybe String
    , vm : Maybe String
    , name : Maybe String
    }

I Get a lot of these List Maybe String 's submatches back from regexing a Http.request text response:

[ Just "0008,0005"
, Just "CS"
, Just "ISO_IR100"
, Nothing
, Just "10"
, Just "1"
, Just "SpecificCharacterSet"
]

I need a way to go from those Lists to those type aliases. I tried:

getResponseLine : List Maybe String -> ResponseLine
getResponseLine =
    foldl (\arg fn -> fn arg) ResponseLine

While there are no errors shown at all in VSCode, reactor cannot build my app, after trying for some time it says:

elm-make: out of memory

I guess some recursive stuff is going on. So, do i need to use Array.fromList and use indexes, or is there a more functional approach i have overlooked? The internet is too full with posts of problems the other way around :( Thanks in advance.

Upvotes: 1

Views: 763

Answers (1)

Luke Woodward
Luke Woodward

Reputation: 64959

If you want to convert a List of 7 Maybe String values to a ResponseLine object, then something like the following should work:

getResponseLine: List (Maybe String) -> ResponseLine
getResponseLine strings =
    case strings of 
        [ tag, vr, value, comments, len, vm, name ] ->
            ResponseLine tag vr value comments len vm name

        _ ->
            Debug.crash "TODO handle errors if list doesn't have 7 items"

I'll leave it up to you to decide what error-handling your need if the list of Maybe String values doesn't have length 7. Note also that I've declared the argument to getResponseLine as of type List (Maybe String) rather than List Maybe String: the compiler will interpret the latter as List being a type that takes two type variables with the values of the type variables being Maybe and String.

I suspect the out-of-memory error is caused while the Elm compiler is trying to figure out types in your call to foldl. It looks related to this Elm compiler bug.

Upvotes: 3

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