# Selectors

The examples run inside an [active graph](graph.fsx).

`createSelector source` is `createLookup` with `affected = fun prev next -> [ prev; next ]`: `Get key` is
`true` for the selected key and `false` for every other. A selection change wakes only the readers of
the previous and the next key.

```fsharp
let selected = createSignal 1
let isSelected = createSelector (fun () -> selected.Value)
selected.Value <- 3
[ for k in 1..4 -> k, isSelected.Get k ]
```

```text
[(1, false); (2, false); (3, true); (4, false)]
```

Move the selection from 1 to 2. Only those two membership values change; the reader for
key 3 stays quiet.

<div class="rv-map-card">

```fsharp
let selected = createSignal 1
let isSelected = createSelector (fun () -> selected.Value)
let first = createMemo (fun _ -> isSelected.Get 1)
let second = createMemo (fun _ -> isSelected.Get 2)
let third = createMemo (fun _ -> isSelected.Get 3)
createEffect (fun () -> printfn "first = %b" first.Value)
createEffect (fun () -> printfn "second = %b" second.Value)
createEffect (fun () -> printfn "third = %b" third.Value)

controls [
    button "Select 2" (fun () -> selected.Value <- 2)
    |> describe "Keys 1 and 2 change their answers; key 3 stays false."
    |> expect "Keys 1 and 2 change their answers; key 3 stays false." (fun () -> not first.Peek && second.Peek && not third.Peek)
    button "Select 3" (fun () -> selected.Value <- 3)
    |> describe "Keys 2 and 3 change their answers; key 1 stays false."
    |> expect "Keys 2 and 3 change their answers; key 1 stays false." (fun () -> not first.Peek && not second.Peek && third.Peek)
]
```

<div class="rv-map-host" id="rvm-p5e22b589f1-c1" data-ranvier-map-page="p5e22b589f1" data-ranvier-map-cell="c1"></div>

</div>


Behavior tests: [Lookups.fs](https://github.com/ranvier-org/Ranvier/blob/master/tests/Ranvier.Tests/Lookups.fs).