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