Create EDSL for manually specifying TOML data types

[chshersh]

Sometimes you don't want to specify TOML in file, you want to have configuration embedded to code. Currently you can specify everything manually using HashMaps and constructors:

myToml :: TOML
myToml = TOML (fromList
    [ (Key "a", Bool True)
    , (Key "list", Array [String "one", String "two"])
    , (Key "time", Array [Date $ Day (fromGregorian 2018 3 29)])
    ] ) myInnerToml mempty

myInnerToml :: HashMap TableId TOML
myInnerToml = fromList $ [ ( TableId (NonEmpty.fromList ["table", "name", "1"])
                           , TOML (fromList [ (Key "aInner", Int 1)
                                            , (Key "listInner", Array [Bool True, Bool False])
                                            ]) myInnerInnerToml mempty
                           )
                         , ( TableId (NonEmpty.fromList ["table", "name", "2"])
                           , TOML (fromList [ (Key "2Inner", Int 42)
                                            ]) mempty mempty
                           )
                         ]

But this is ugly and not convenient. What would be great is to have some EDSL so the above can be configured like this:

myToml :: TOML
myToml = configure $ do
  "a"    :=: True
  "list" :=: ["one", "two"]
  "time" :=: Day (fromGregorian 2018 3 29) -- maybe something more convenient here
  table "table.name.1" $ do
    "aInner"    :=: 1
    "listInner" :=: [True, False]
    ...

The last one looks more convenient and simpler to me.