6.1.1. Controlling extensions

Language extensions can be controlled (i.e. allowed or not) in two ways:

• Every language extension can be switched on by a command-line flag “-X...” (e.g. -XTemplateHaskell), and switched off by the flag “-XNo...”; (e.g. -XNoTemplateHaskell).

• Language extensions can also be enabled using the LANGUAGE pragma, thus {-# LANGUAGE TemplateHaskell #-} (see LANGUAGE pragma).

Haskell2010

Compile Haskell 2010 language variant. Enables the following language extensions:

ImplicitPrelude StarIsType CUSKs MonomorphismRestriction DatatypeContexts TraditionalRecordSyntax

EmptyDataDecls ForeignFunctionInterface PatternGuards DoAndIfThenElse RelaxedPolyRec

Haskell98

Compile using Haskell 98 language variant. Enables the following language extensions:

ImplicitPrelude StarIsType CUSKs MonomorphismRestriction

NPlusKPatterns DatatypeContexts TraditionalRecordSyntax NondecreasingIndentation

Although not recommended, the deprecated -fglasgow-exts flag enables a large swath of the extensions supported by GHC at once.

-fglasgow-exts

The flag -fglasgow-exts is equivalent to enabling the following extensions:

ConstrainedClassMethods DeriveDataTypeable DeriveFoldable DeriveFunctor DeriveGeneric DeriveTraversable EmptyDataDecls ExistentialQuantification ExplicitNamespaces FlexibleContexts FlexibleInstances ForeignFunctionInterface FunctionalDependencies GeneralizedNewtypeDeriving ImplicitParams InterruptibleFFI

KindSignatures LiberalTypeSynonyms MagicHash MultiParamTypeClasses ParallelListComp PatternGuards PostfixOperators RankNTypes RecursiveDo ScopedTypeVariables StandaloneDeriving TypeOperators TypeSynonymInstances UnboxedTuples UnicodeSyntax UnliftedFFITypes

Enabling these options is the only effect of -fglasgow-exts. We are trying to move away from this portmanteau flag, and towards enabling features individually.