Tired of including a long list of LANGUAGE pragmas at the top of
your Haskell source files? This is a preprocessor that expands
shorthand for collections of common pragmas. The only currently
defined meta-pragma is Haskell2015 as a guess to what people might
expect to work out of the box today. It includes
- ConstraintKinds
- DataKinds
- EmptyCase
- FlexibleContexts
- FlexibleInstances
- GADTs
- InstanceSigs
- KindSignatures
- LambdaCase
- MultiParamTypeClasses
- NoImplicitPrelude
- RankNTypes
- ScopedTypeVariables
- TypeFamilies
- TypeOperators
You use this preprocessor by installing it somewhere (I build it in a
sandbox, and cabal install --symlink-bindir ~/.cabal/bin), then, in
the source file you want to use it, you write,
{-# META Haskell2015 #-}
{-# OPTIONS_GHC -F -pgmF metapragma #-}You can include other LANGUAGE pragmas on other lines at the top of
the file.
If you want to define your own meta-pragmas, create a directory
$HOME/.metapragma and place within it plain text files with one
LANGUAGE pragma per line. You can then refer to this meta-pragma by
the name of the file (any file extension will be dropped).
To reduce boilerplate. This saves a small amount of time, and reduces the vagaries of how people manage long lists of pragmas.
You can list language extension pragmas in your .cabal file, but
there is a preference among some for including this information in the
source file to reduce reader surprise, and play better with
stand-alone file reading (e.g. as a blog post). While, today, nobody
knows what Haskell2015 as used here implies, the intent is to offer
a straw man that folks can rally around or beat to the ground in order
to nail down a set of commonly used meta-pragmas (or maybe just one a
year).