Keyboard signals and foldp in Elm -

i'm trying make text field submits input pressing enter , cleared 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 empty field while holding escape , reverts whatever entered when escape released.

trying understand why case lead me 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 emptied on pressing escape releasing results in string of single (last entered) character.

from understand, keyboard.isdown signal triggered while holding , on key release. how can clear field persistently?

the reason see behaviour

when create signal of pairs (signal (keycode,bool)) out of 2 signals (signal keycode/signal bool), signal of pairs update every time of of signals update. value of signal.map2 (,) keyboard.presses escapedown on time might be:

(0,false), (97,false), (98,false), (98,true), (98,false) ^          ^ press 'a' ^ press 'b'  ^          ^ release escape program start                       press escape 

when press escape, pair of values changes , foldp updates empty string. when release escape pair of values changes again, foldp updates again, finds false value, therefore appends last character pressed empty string.

solution

in case you're interested in event when escape pressed, not when isdown. instead of creating pair of signals, in case better merge signals. need of same type. here's 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 = 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 union type input, represent different inputs program. presses wrapped in keypress constructor of union type. escape button represented other constructor escapepress. have 2 signals of same type, can merge. in step function pattern match on constructors of input, , handle familiar cases.
note i'm filtering keyboard.presses signal don't keypress event pressing down, holding down or letting go of escape key.