Idiomatic Means to Rewrite Haskell Json Creating Library-CodePudding

I do not wish to be rude in asking this question; however, I have recently been attempting to learn Haskell and wish to understand the best practices of the language. I am currently reading about a library that parses a Haskell file and emits a parse tree as a json document. The contents of Main.hs appear repetitive and attempt to derive a json producer for every element of the parse tree. Do people commonly use code generators when writing Haskell programs? I would assume that computer created code would defeat the elegance of functional programming, or am I wrong? Would there be any idiomatic means of simplifying the following code?

// ~80 lines of the same statements have been elided
instance ToJSON l => ToJSON (FunDep l) where
    toJSON (FunDep l n1 n2)
        = object [pack "FunDep" .= [toJSON l, toJSON n1, toJSON n2]]

instance ToJSON l => ToJSON (GadtDecl l) where
    toJSON (GadtDecl l n1 t2)
        = object [pack "GadtDecl" .= [toJSON l, toJSON n1, toJSON t2]]

instance ToJSON l => ToJSON (FieldDecl l) where
    toJSON (FieldDecl l n1 b2)
        = object [pack "FieldDecl" .= [toJSON l, toJSON n1, toJSON b2]]

Given that all of the functions possess similar signatures, would it not be possible to create a function builder that returns the necessary parameters for each element of the parse tree? Or, could one at least store the necessary information in a csv file and retrieve it at runtime?

I am very enthusiastic regarding the continuation of my Haskell studies and do not wish to single out any particular repository.

CodePudding user response：

Generating code is normal, idiomatic, and used in exactly the file you pointed at. If you scroll up a few lines, you'll see:

-- FIXME: Waiting for https://github.com/bos/aeson/issues/206.
{-
-- ~15 lines of the same statements have been elided
$(deriveToJSON defaultOptions { sumEncoding = ObjectWithSingleField } ''FunDep)
$(deriveToJSON defaultOptions { sumEncoding = ObjectWithSingleField } ''GadtDecl)
$(deriveToJSON defaultOptions { sumEncoding = ObjectWithSingleField } ''FieldDecl)
-}

These lines are intended, at some point in the future, to expand to the ones you show; see the linked issue for more on why they don't currently.

(Aside: there is nothing stopping somebody from creating a deriveToJSONs or something that takes a collection of type names instead of a single type name. They probably should do that, so that they could write

deriveToJSONs defaultOptions { sumEncoding = ObjectWithSingleField }
    [''FunDep, ''GadtDecl, ''FieldDecl]

and just have a list with ~50 elements rather than 50 lines of deriving calls. Probably they just didn't think of it.)