purescript struggles

footpedal_vp.purs

module Main where

import Control.Monad.Eff
import Data.Array

import Debug.Trace

main = trace "Hello, World!"

service dev endpoint = do
  trace "awaiting connection..."
  handle { target: endpoint, tag: "connection", handler: \conn -> do
                trace "got connection!"
                trace "awaiting foot pedal event..."
                handle { target: dev, tag: "data", handler: \chunk -> do
                        let event = decode chunk
                        trace "@@$ serialize event"
                        conn.send serialize event
                          }}

foreign import serialize
  "function serialize(obj) { return JSON.stringify(obj) }" :: {} -> String

foreign import data WindowIO :: !

foreign import handle
  "function handle(it) { it.target.on(it.tag, it.handler) }" ::
    forall t h e. { target:: t,
                    tag:: String,
                    handler:: h } -> Eff (io :: WindowIO | e) {}

type PedalState = {pedal:: Number, pressed:: Number}

decode :: [Number] -> [PedalState]
decode chunk = each <$> [0, 1, 2]
               where each ix = { pedal: chunk !! (ix * 8),
                                 pressed: chunk !! (ix * 8 + 4) }

travesable_issue.purs

module Footpedal where

import Data.Array
import Data.Maybe
import Data.Traversable
import Data.Tuple
import Control.Applicative

import Debug.Trace

main = trace "Hello, World!"

type PedalState = {pedal:: Number, pressed:: Number}

decode :: [Number] -> Maybe [PedalState]
decode bytes = sequence $ do
  ix <- range 0 2
  let offset = ix * 8
  return $ parts offset
  where
    parts :: Number -> Maybe PedalState
    parts offset = do
                 pedal_num <- bytes !! offset
                 state <- bytes !! (offset + 4)
                 return { pedal: pedal_num, pressed: state }

>> Error at src/footpedal_vp.purs line 16, column 16: 
>> Error in declaration decode
>> Attempted to unify a constrained type (Prelude.Functor u37, Prelude.Applicative u37) => u36 (u37 u38) -> u37 (u36 u38) with another type.