The documentation for template Haskell says that QuasiQuoter is defined as
data QuasiQuoter = QuasiQuoter { quoteExp :: String -> Q Exp,
quotePat :: String -> Q Pat,
quoteType :: String -> Q Type,
quoteDec :: String -> Q [Dec] }
I have encountered examples such as silly shown below. How do they work given that they don't seem to fill in quotePat, quoteType, and quoteDec fields?
silly :: QuasiQuoter
silly = QuasiQuoter { quoteExp = \_ -> [| "yeah!!!" |] }
The answer has nothing to do with TH: if you don't initialize a field of a record constructor, trying to access that field will cause an error:
>data D = D {a :: Int, b :: Char} deriving Show
>D {a = 0}
D {a = 0, b = *** Exception: <interactive>:26:1-9: Missing field in record construction :Interactive.b
In this case, this is desired behavior since certain quoters only make sense in a certain context, and should throw an error if they are used improperly. Although, you would most likely want to provide more descriptive error message for your quoters, like so:
quote = QuasiQuoter {
quoteExp = something,
quotePat = error "quote: Invalid application in pattern context." ....