parseModule does not recognize some language extensions as enabled

Question

How do I make parseModule parse Haskell files with language extensions?

Using parseModule from Language.Haskell.Exts, when I try to parse the file Core.hs from https://github.com/xmonad/xmonad/blob/master/src/XMonad/Core.hs I get the error:

XGene-exe: fromParseResult: Parse failed at [<unknown>.hs] (248:25): Illegal data/newtype declaration

This appears to be because it is using an existential type:

data Layout a = forall l. (LayoutClass l a, Read (l a)) => Layout (l a)

yet Core.hs has the ExistentialQuantification language extension pragma at the top:

{-# LANGUAGE ExistentialQuantification, FlexibleInstances, GeneralizedNewtypeDeriving,
             MultiParamTypeClasses, TypeSynonymInstances, DeriveDataTypeable #-}

when I try it for Layout.hs (https://github.com/xmonad/xmonad/blob/master/src/XMonad/Core.hs) I get the error Parse failed at [<unknown>.hs] (53:1): MultiParamTypeClasses language extension is not enabled. Please add {-# LANGUAGE MultiParamTypeClasses #-} pragma at the top of your module. despite there pragma being there:

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, PatternGuards, TypeSynonymInstances, DeriveDataTypeable #-}

Main.hs and Operations.hs gives the error Malformed context: FlexibleContexts is not enabled despite having {-# LANGUAGE MultiParamTypeClasses, FlexibleContexts #-}

Config.hs, ManageHook.hs and Stackset.hs parse correctly.

xmonad builds when I do a cabal build.

Answer 1

Thanks to the Guidance of duplode's comment, the issue was that I was using parseModule, instead of parseFile.

parseFile automatically picks up the language extensions, whereas if I wanted to parse just the source code then I would have to use parseModuleWithMode and add the relevant extensions.

parseFile fits my use case better.