{
    {-# OPTIONS_GHC -fno-warn-name-shadowing #-}
    {-# LANGUAGE DeriveGeneric #-}
    {-# LANGUAGE DeriveAnyClass #-}
    {-# LANGUAGE StandaloneDeriving #-}
    {-# LANGUAGE OverloadedStrings #-}
    {-# LANGUAGE FlexibleContexts #-}

    module Language.Dhall.Lexer.Mod
        ( lexDhall
        , alexMonadScan
        , runAlex
        , Alex (..)
        , AlexPosn (..)
        , Ann (..)
        , Token
        , AlexState (..)
        ) where

import Control.Arrow ((&&&))
import Control.DeepSeq (NFData)
import Control.Monad
import Data.Bool (bool)
import qualified Data.ByteString.Lazy as BSL
import qualified Data.ByteString.Lazy.Char8 as ASCII
import Data.Scientific
import GHC.Generics (Generic)
import GHC.Natural (Natural)
import Language.Dhall.Lexer.Types

}

%wrapper "monad-bytestring"

$digit = 0-9

@index = $digit+
@natural = \+ $digit+
@integer = (\- | "") $digit+
@double = (\- | "") $digit+ \. $digit+

-- TODO // for lambdas?
$special = [\{\}\,\=\:\[\]\<\>\|\(\)\.\+\*\#]

$lowercase = [a-z]
$uppercase = [A-Z]
$letter = [$lowercase $uppercase]

@identifier_body = $letter | $digit | _

@type = $uppercase @identifier_body*
@quoted = \` @type \`
@identifier = ($lowercase | _) @identifier_body*

@url_loc = "http://" | "https://"
@file_loc = "./" | "../" | "/"

$url_contents = [\:\/\-\.\_ $letter $digit]

@url = @url_loc $url_contents+
@file = @file_loc $url_contents+

$string_char = $printable # [\"\\\$]

$esc_char = [\"\$\\]

-- Deficiency: what if $ is followed by "?
@string_in = (\\ $esc_char | $string_char | \$ [^\{\"])*

tokens :-

    <0,splice> $white+           ;

    <0,splice> "--".*            ;
    "{-"                         { \_ _ -> nested_comment }

    -- keywords
    <0,splice> if                { tok (\p _ -> alex p $ Keyword KwIf) }
    <0,splice> then              { tok (\p _ -> alex p $ Keyword KwThen) }
    <0,splice> else              { tok (\p _ -> alex p $ Keyword KwElse) }
    <0,splice> let               { tok (\p _ -> alex p $ Keyword KwLet) }
    <0,splice> in                { tok (\p _ -> alex p $ Keyword KwIn) }
    <0,splice> forall            { tok (\p _ -> alex p $ Keyword KwForall) }
    <0,splice> "∀"               { tok (\p _ -> alex p $ Keyword KwForall) }
    <0,splice> constructors      { tok (\p _ -> alex p $ Keyword KwConstructors) }
    <0,splice> merge             { tok (\p _ -> alex p $ Keyword KwMerge) }
    <0,splice> Text              { tok (\p _ -> alex p $ Keyword KwText) }
    <0,splice> Double            { tok (\p _ -> alex p $ Keyword KwDouble) }
    <0,splice> Integer           { tok (\p _ -> alex p $ Keyword KwInteger) }
    <0,splice> List              { tok (\p _ -> alex p $ Keyword KwList) }
    <0,splice> Natural           { tok (\p _ -> alex p $ Keyword KwNatural) }
    <0,splice> Bool              { tok (\p _ -> alex p $ Keyword KwBool) }
    <0,splice> Optional          { tok (\p _ -> alex p $ Keyword KwOptional) }
    <0,splice> "Nat/fold"        { tok (\p _ -> alex p $ Keyword KwNatFold) }
    <0,splice> "Integer/show"    { tok (\p _ -> alex p $ Keyword KwIntegerShow) }
    <0,splice> Kind              { tok (\p _ -> alex p $ Keyword KwKind) }
    <0,splice> Type              { tok (\p _ -> alex p $ Keyword KwType) }

    -- builtin specials
    <0,splice> "//"              { tok (\p _ -> alex p $ Operator CombineTok) }
    <0,splice> "⫽"               { tok (\p _ -> alex p $ Operator CombineTok) }
    <0,splice> "/"\\             { tok (\p _ -> alex p $ Operator PreferTok) }
    <0,splice> "∧"               { tok (\p _ -> alex p $ Operator PreferTok) }
    <0,splice> "→"               { tok (\p _ -> alex p $ Operator ArrowTok) }
    <0,splice> "->"              { tok (\p _ -> alex p $ Operator ArrowTok) }
    <0,splice> "λ"               { tok (\p _ -> alex p $ Operator LambdaTok) }
    <0,splice> \\                { tok (\p _ -> alex p $ Operator LambdaTok) }
    <0,splice> \&\&              { tok (\p _ -> alex p $ Operator AndTok) }
    <0,splice> \|\|              { tok (\p _ -> alex p $ Operator OrTok) }
    <0,splice> \=\=              { tok (\p _ -> alex p $ Operator EqTok) }
    <0,splice> \!\=              { tok (\p _ -> alex p $ Operator NeqTok) }
    <0,splice> \+\+              { tok (\p _ -> alex p $ Operator AppendTok) }

    -- Path literals
    <0,splice> @url              { tok (\p s -> alex p $ EmbedURL s) }
    <0,splice> @file             { tok (\p s -> alex p $ EmbedFile s) }

    -- Various special characters
    <0> $special                 { tok (\p s -> alex p $ Special s) }

    -- Numeric literals
    <0,splice> @double           { tok (\p s -> Ann p <$> fmap DoubleTok (readDouble s)) }
    <0,splice> @natural          { tok (\p s -> Ann p <$> fmap NatLit (readNatural s)) }
    <0,splice> \@ @index         { tok (\p s -> Ann p <$> fmap AtToken (readInteger (BSL.tail s))) }
    <0,splice> @integer          { tok (\p s -> Ann p <$> fmap IntLit (readInteger s)) }

    -- Boolean literals
    <0,splice> True              { tok (\p _ -> alex p $ BoolTok True) }
    <0,splice> False             { tok (\p _ -> alex p $ BoolTok False) }

    -- Identifiers
    <0,splice> @quoted           { tok (\p s -> alex p $ QuotedId s) }
    <0,splice> @identifier       { tok (\p s -> alex p $ Identifier s) }
    <0,splice> @type             { tok (\p s -> alex p $ TypeId s) }

    -- Strings & string splices
    <0,splice> \"                { begin string }
    <string> @string_in \$ / \"  { tok (\p s -> alex p $ StringChunk s) }
    <string> @string_in          { tok (\p s -> alex p $ StringChunk s) }
    <string> \$\{                { tok (\p _ -> alex p $ BeginSplice) `andBegin` splice }
    <splice> \}                  { tok (\p _ -> alex p $ EndSplice) `andBegin` string }
    <splice> $special # \}       { tok (\p s -> alex p $ Special s) }
    <string> \"                  { begin 0 }

{

-- Taken from example by Simon Marlow.
nested_comment :: Alex Token
nested_comment = go 1 =<< alexGetInput

    where go :: Int -> AlexInput -> Alex Token
          go 0 input = alexSetInput input >> alexMonadScan
          go n input =
            case alexGetByte input of
                Nothing -> err input
                Just (c, input') ->
                    case Data.Char.chr (fromIntegral c) of
                        '-' ->
                            case alexGetByte input' of
                                Nothing -> err input'
                                Just (125,input_) -> go (n-1) input_
                                Just (_,input_) -> go n input_
                        '{' ->
                            case alexGetByte input' of
                                Nothing -> err input'
                                Just (c',input_) -> go (addLevel c' $ n) input_
                        _ -> go n input'

          addLevel c' = bool id (+1) (c'==45)

          err (pos,_,_,_) =
            let (AlexPn _ line col) = pos in
                alexError ("Error in nested comment at line " ++ show line ++ ", column " ++ show col)

readNatural :: BSL.ByteString -> Alex Natural
readNatural = go <=< readInteger . BSL.tail
    where go x | x < 0 = alexError "Internal lexer error"
          go x = pure (fromIntegral x)

readDouble :: BSL.ByteString -> Alex Scientific
readDouble str = pure $ read (ASCII.unpack str)

readInteger :: BSL.ByteString -> Alex Integer
readInteger str =
    case ASCII.readInteger str of
        Just (i, "") -> pure i
        _ -> alexError "Not a valid integer"

get_pos :: Alex AlexPosn
get_pos = Alex (Right . (id &&& alex_pos))

alex :: AlexPosn -> a -> Alex (Ann AlexPosn a)
alex = (pure .) . Ann

tok f (p,_,s,_) len = f p (BSL.take len s)

deriving instance Generic AlexPosn
deriving instance NFData AlexPosn

-- | Data type for values with annotations.
data Ann a b = Ann { loc   :: a
                   , inner :: b
                   }
           deriving (Eq, Show, Generic, NFData)

-- | A token with location information.
type Token = Ann AlexPosn TokenType

alexEOF :: Alex Token
alexEOF = Ann <$> get_pos <*> pure End

lexDhall :: BSL.ByteString -> Either String [Token]
lexDhall str = runAlex str loop

loop :: Alex [Token]
loop = do
    tok' <- alexMonadScan
    if inner tok' == End then pure mempty
        else (tok' :) <$> loop

}