Which Haskell (GHC) extensions should users use/avoid?

Question

I have had the experience a few times now of having GHC tell me to use an extension, only to discover that when in using that extension I have made code far more complex whe

Answer 1

An ad hoc list of morally "good" extensions, and morally "bad" ones - this is an aesthetic judgement!

The Good

• GADTs
• Parallel list comprehensions
• Pattern guards
• Monad comprehensions
• Tuple sections
• Record wild cards
• Empty data decls
• Existential types
• Generalized new type deriving
• MPTCs + FDs
• Type families
• Explicit quantification
• Higher rank polymorphism
• Lexically scoped tyvars
• Bang Patterns

The Bad

• SQL comprehensions
• Implicit parameters

The Ugly (but necessary)

• Template Haskell
• Unboxed types and tuples
• Undecidable, overlapping and incoherent instances -- usually means you have a misdesign.

Not sure

• Arrow notation
• View patterns