elm/step1_read_print.elm

   1 port module Main exposing (..)
   2
   3 import IO exposing (..)
   4 import Json.Decode exposing (decodeValue)
   5 import Platform exposing (programWithFlags)
   6 import Types exposing (MalExpr(..))
   7 import Reader exposing (readString)
   8 import Printer exposing (printStr)
   9
  10
  11 main : Program Flags Model Msg
  12 main =
  13     programWithFlags
  14         { init = init
  15         , update = update
  16         , subscriptions =
  17             \model -> input (decodeValue decodeIO >> Input)
  18         }
  19
  20
  21 type alias Flags =
  22     { args : List String
  23     }
  24
  25
  26 type alias Model =
  27     { args : List String
  28     }
  29
  30
  31 type Msg
  32     = Input (Result String IO)
  33
  34
  35 init : Flags -> ( Model, Cmd Msg )
  36 init flags =
  37     ( flags, readLine prompt )
  38
  39
  40 update : Msg -> Model -> ( Model, Cmd Msg )
  41 update msg model =
  42     case msg of
  43         Input (Ok (LineRead (Just line))) ->
  44             case rep line of
  45                 Just out ->
  46                     ( model, writeLine out )
  47
  48                 Nothing ->
  49                     ( model, readLine prompt )
  50
  51         Input (Ok LineWritten) ->
  52             ( model, readLine prompt )
  53
  54         Input (Ok (LineRead Nothing)) ->
  55             ( model, Cmd.none )
  56
  57         Input (Ok io) ->
  58             Debug.crash "unexpected IO received: " io
  59
  60         Input (Err msg) ->
  61             Debug.crash msg ( model, Cmd.none )
  62
  63
  64 prompt : String
  65 prompt =
  66     "user> "
  67
  68
  69 {-| read can return three things:
  70
  71 Ok (Just expr) -> parsed okay
  72 Ok Nothing -> empty string (only whitespace and/or comments)
  73 Err msg -> parse error
  74
  75 -}
  76 read : String -> Result String (Maybe MalExpr)
  77 read =
  78     readString
  79
  80
  81 eval : MalExpr -> MalExpr
  82 eval ast =
  83     ast
  84
  85
  86 print : MalExpr -> String
  87 print =
  88     printStr True
  89
  90
  91 {-| Read-Eval-Print
  92 -}
  93 rep : String -> Maybe String
  94 rep =
  95     let
  96         formatResult result =
  97             case result of
  98                 Ok optStr ->
  99                     optStr
 100
 101                 Err msg ->
 102                     Just msg
 103     in
 104         readString
 105             >> Result.map (Maybe.map (eval >> print))
 106             >> formatResult