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POUsing Trifecta's layout parser
 

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    <p>I'm experimenting with Trifecta for parsing a very simple functional language with Haskell-like layout syntax. I'm working off the Haddock docs and my experience with Parsec, because I couldn't find any introductory material on Trifecta itself. </p> <p>The problem I'm having is with using the layout stuff, since not even the Haddock docs help much. </p> <p>Given the following code:</p> <pre><code>import Text.Trifecta import Text.Trifecta.Parser.Token.Style import Text.Trifecta.Parser.Identifier.Style import Text.Trifecta.Layout.Combinators import Text.Trifecta.Language.Prim import Control.Applicative import Control.Monad.Trans import Data.Maybe (fromMaybe) import Data.HashSet as HashSet import Data.ByteString.UTF8 as UTF8 -- Copypasta from Text.Trifecta.Parser.Identifier.Style set :: [String] -&gt; HashSet ByteString set = HashSet.fromList . fmap UTF8.fromString lang :: MonadParser m =&gt; LanguageDef m lang = LanguageDef{ languageCommentStyle = haskellCommentStyle , languageIdentifierStyle = emptyIdents{ styleReserved = set keywords } , languageOperatorStyle = emptyOps{ styleReserved = set ops } } where keywords = ["where"] ops = ["="] data Def = Def ByteString [ByteString] [ByteString] [Def] deriving Show instance MonadLanguage m =&gt; MonadLanguage (Layout m) where askLanguage = fmap liftLanguageDef $ lift askLanguage def :: (MonadParser m) =&gt; Layout (Language m) Def def = Def &lt;$&gt; identifier &lt;*&gt; vars &lt;* reservedOp "=" &lt;*&gt; vars &lt;*&gt; laidout locals where vars = many identifier locals = fromMaybe [] &lt;$&gt; optional (reserved "where" *&gt; defs) defs :: (MonadParser m) =&gt; Layout (Language m) [Def] defs = laidout (many def) test :: String -&gt; IO () test = parseTest $ run $ defs &lt;* eof where run p = runLanguage (fst &lt;$&gt; runLayout p defaultLayoutState) lang </code></pre> <p>I'm trying to parse the following text with <code>test</code>:</p> <pre><code>f x = x y a b c -- 1 where -- 2 y = d -- 3 g x = z -- 4 </code></pre> <p>but it fails with this parse error:</p> <pre><code>(interactive):4:2: error: expected: "=", identifier, letter or digit g x = z -- 4 ^ </code></pre> <p>but if I comment out lines 2 and 3, it works.</p> <p>So how do I make it parse even when including lines 2 and 3?</p>
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