Record syntax for QuasiQuoter constructor

Question

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!!!" |] } ```

Accepted Answer

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." ....
```

Record syntax for QuasiQuoter constructor

Solution

Problem