Keyboard signals and foldp in Elm - elm

Question

I'm trying to make a text field that submits its input by pressing Enter and is cleared by Escape.
import Graphics.Element exposing (flow, down, show)
import Signal exposing ((<~), (~))
import Graphics.Input.Field exposing (noContent, defaultStyle, field)
import String
import Keyboard
main = Signal.map view (Signal.foldp step init signals)
signals = (,,) <~ box.signal
~ Signal.sampleOn Keyboard.enter box.signal
~ escape
escape = Keyboard.isDown 27
box = Signal.mailbox noContent
init = (noContent, noContent, False)
view (text, query, clear) =
flow down
[ field defaultStyle (Signal.message box.address) "Enter text" text
, show (String.words query.string)
]
step (text, query, clear) _ =
case clear of
False -> (text, query, clear)
True -> (noContent, noContent, clear)
This produces an empty field only while holding Escape and reverts to whatever was entered when Escape is released.
Trying to understand why this is the case lead me to a smaller example:
import Graphics.Element exposing (show)
import Signal
import Char
import String
import Keyboard
main = Signal.map show input
input = Signal.foldp step "" (Signal.map2 (,) Keyboard.presses escapeDown)
escapeDown = Keyboard.isDown 27
step (keyCode, esc) string =
case esc of
True -> ""
False -> string ++ keyToString keyCode
keyToString = String.fromChar << Char.fromCode
The accumulated string of characters is emptied on pressing Escape but releasing it results in a string of a single (last entered) character.
From what I understand, Keyboard.isDown signal is triggered while holding and on key release. So how can I clear the field persistently?

Answer 1

The reason you see this behaviour
When you create a signal of pairs (Signal (KeyCode,Bool)) out of two signals (Signal KeyCode/Signal Bool), that signal of pairs will update every time of of the signals update. So the value of Signal.map2 (,) Keyboard.presses escapeDown over time might be:
(0,False), (97,False), (98,False), (98,True), (98,False)
^ ^ press 'a' ^ press 'b' ^ ^ release escape
program start press escape
When you press escape, the pair of values changes and your foldp updates to the empty string. When you release escape the pair of values changes again, so foldp updates again, finds the False value, therefore appends the last character you pressed to the empty string.
Solution
In this case you're really interested in the event when escape is pressed, but not when it isDown. Instead of creating a pair of the signals, it is in this case better to merge the signals. To do so they need to be of the same type. Here's an example:
import Graphics.Element exposing (show)
import Signal exposing ((<~))
import Char
import String
import Keyboard
type Input = KeyPress Keyboard.KeyCode | EscapePress
main = Signal.map show output
presses = KeyPress <~ Signal.filter ((/=) 27) 0 Keyboard.presses
escapePress = always EscapePress <~ escapeDown
input = Signal.merge escapePress presses
output = Signal.foldp step "" input
escapeDown = Keyboard.isDown 27
step input string =
case input of
EscapePress -> ""
KeyPress keyCode -> string ++ keyToString keyCode
keyToString = String.fromChar << Char.fromCode
With a union type Input, we represent the different inputs to the program. The presses are wrapped in the KeyPress constructor of the union type. The escape button is represented with the other constructor EscapePress. Now you have two signals of the same type, which you can merge. In your step function you pattern match on the constructors of your Input, and handle the familiar cases.
Note that I'm filtering the Keyboard.presses signal so you don't get a KeyPress event from pressing down, holding down or letting go of the escape key.