Purescript types for buildQueryString function - purescript

Question

I am new to Purescript and I am trying to write a function that
can take any record value and iterate over the fields and values and build
a querystring.
I am thinking something like:
buildQueryString :: forall a. PropertyTraversible r => r -> String
which I want to use like this:
buildQueryString {name: "joe", age: 10} -- returns: "name=joe&age=10"
Is there a way to write something like that in Purescript with existing idioms or do I have to create my own custom Type Class for this?

Answer 1

I'm sure that it can be shorter, but here is my implementation based on purescript-generic-rep (inspired by genericShow). This solution uses typeclasses - it seems to be standard approach with generic-rep:
module Main where
import Prelude
import Control.Monad.Eff (Eff)
import Control.Monad.Eff.Console (CONSOLE, log)
import Data.Foldable (intercalate)
import Data.Generic.Rep (class Generic, Constructor(..), Field(..), Product(..), Rec(..), from)
import Data.Symbol (class IsSymbol, SProxy(..), reflectSymbol)
class EncodeValue a where
encodeValue ∷ a → String
instance encodeValueString ∷ EncodeValue String where
encodeValue = id
instance encodeValueInt ∷ EncodeValue Int where
encodeValue = show
class EncodeFields a where
encodeFields :: a -> Array String
instance encodeFieldsProduct
∷ (EncodeFields a, EncodeFields b)
⇒ EncodeFields (Product a b) where
encodeFields (Product a b) = encodeFields a <> encodeFields b
instance encodeFieldsField
∷ (EncodeValue a, IsSymbol name)
⇒ EncodeFields (Field name a) where
encodeFields (Field a) =
[reflectSymbol (SProxy :: SProxy name) <> "=" <> encodeValue a]
buildQueryString
∷ ∀ a l n.
Generic n (Constructor l (Rec a))
⇒ (EncodeFields a)
⇒ n
→ String
buildQueryString n =
build <<< from $ n
where
build (Constructor (Rec fields)) = intercalate "&" <<< encodeFields $ fields
newtype Person =
Person
{ name ∷ String
, age ∷ Int
}
derive instance genericPerson ∷ Generic Person _
joe ∷ Person
joe = Person { name: "joe", age: 10 }
main :: forall e. Eff (console :: CONSOLE | e) Unit
main = do
log <<< buildQueryString $ joe
buildQueryString expects value of type with single constructor which contains a record (possibly just newtype) because it is impossible to derive a Generic instance for "unwrapped" Record type.
If you want to handle also Array values etc. then encodeValue should probably return values of type Array String.

Answer 2

This is possible with purescript-generics but it only works on nominal types, not on any record. But it saves you boilerplate, since you can just derive the instance for Generic, so it would work with any data or newtype without further modification.
Downside is, you have to make some assumptions about the type: like it only contains one record and the record does not contain arrays or other records.
Here is a hacky demonstration how it would work:
data Person = Person
{ name :: String
, age :: Int
}
derive instance genericPerson :: Generic Person
joe = Person { name: "joe", age: 10 }
build :: GenericSpine -> String
build (SRecord arr) = intercalate "&" (map (\x -> x.recLabel <> "=" <> build (x.recValue unit)) arr)
build (SProd _ arr) = fromMaybe "TODO" $ map (\f -> build (f unit)) (head arr)
build (SString s) = s
build (SInt i) = show i
build _ = "TODO"
test = build (toSpine joe)
purescript-generics-rep is newer, so possibly there is a better solution, maybe even on any record. I have not tried that (yet).