There are some GHC extensions that are too good to live without. Among my favorites are
- Multiparameter type classes
- Scoped type variables
- Higher-rank types
- Generalized algebraic data types (GADTs)
Of these the really essential one is multiparameter type classes.
Some GHC extensions are very speculative and experimental, and you may want to use with caution. A good way to identify a stable and trusted extension is to see if it is slated for inclusion in Haskell Prime, which is hoped to be the successor the Haskell 98.
I second Don Stewart’s suggestion that every extension should be marked using the LANGUAGE pragma in the source file. Don’t enable extensions using command-line options.