How to write pattern splices in expression quotation?

Question

I'm trying to write a declaration using expression quotation, and whatever I try the compiler fails on pattern with a message like the following:

Parse error in pattern: $pattern

Here's an example:

{-# LANGUAGE TemplateHaskell, QuasiQuotes #-}
import Language.Haskell.TH

data A = A Int Int

decl :: DecsQ
decl = [d|
    instance Show A where
      show $pattern = undefined
  |]
  where
    pattern = conP (mkName "A") $ map varP $ map mkName $ ["a", "b"]

Answer 1

The TH documentation says that:

Note that pattern splices are not supported

More details here

Answer 2

GHC HEAD (7.8-to-be) has improved TH support and successfully compiles your code.