pattern matching on a field within a record

[Martin DeMello]

If I have the following code:

type Cell = Empty | Letter String

type alias Square =

  { cell : Cell

  , num : Int

  } 

and c : Maybe Square,

can I do the following in a single match?

 

  case c of

    Just { cell } ->

      case cell of

        Letter s -> s

        _ -> ""

    _ -> ""

that is, i conceptually want

  case c of

    Just { cell = Letter s } -> s

    _ -> ""

martin

[Max Goldstein]

I don't think there's a more compact way to do that, no.

[Simon]

This should work

 

 case c of

    Just (Letter s) ->

      s

    _ -> ""

[Martin DeMello]

Don't think that will work because c is of type Maybe Square, not Maybe Cell.

One further question - if I define

let null_square = { cell = Empty, num = 0 }

let get_square maybe_square =

  case maybe_cell of

    Nothing: null_square

    Just s : s

let get_letter maybe_square =

  case maybe_square of

    { Letter s } -> s

    _ -> ""

would that be introducing an inefficient creation of a null_square every time I call get_letter or is it just a cheap reference?

martin

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[Nick H]

Re: your initial post... you could rewrite the code as

case Maybe.map .cell c of

  Just (Letter s) ->

    s

  _ ->
    ""

Don't know if that's any easier to read, though.

Regarding null_square: If you want to make sure that null_square is passed by reference, you can define it as a top-level value. (That said, the record is so small, I doubt it would make a noticeable performance difference, unless you're planning on making a gazillion of them.)
 

[Martin DeMello]

On Sat, Sep 24, 2016 at 9:11 PM, Nick H <falling...@gmail.com> wrote:

Regarding null_square: If you want to make sure that null_square is passed by reference, you can define it as a top-level value. (That said, the record is so small, I doubt it would make a noticeable performance difference, unless you're planning on making a gazillion of them.)

That's what I ended up doing, and the code was pretty nice with it.

martin