No type class instance was found

Help
#1

Hello everybody,

I’m experimenting with typesafe routing and compiling of this code:

module Routing where

import Type.Proxy (Proxy(Proxy))

import Data.Symbol (class IsSymbol, SProxy(SProxy), reflectSymbol)

import Record (insert, get)

data Param (label :: Symbol) ty = Param

class ParseParam a where
  parseParam :: String -> Maybe a
  renderParam :: a -> String

class CreatePath xs params where
  createPath :: Proxy xs -> Record params -> String

instance paramCreatePath :: ( IsSymbol label, ParseParam ty ) => CreatePath (Param label ty) params where
  createPath _ params = renderParam (get (SProxy :: SProxy label) params)

results in this error:

No type class instance was found for
                         
    Routing.ParseParam t0
                         
  The instance head contains unknown type variables. Consider adding a type annotation.

while applying a function renderParam
  of type ParseParam t0 => t0 -> String
  to argument (get SProxy) params
while checking that expression renderParam ((get SProxy) params)
  has type String
in value declaration paramCreatePath

where t0 is an unknown type

See https://github.com/purescript/documentation/blob/master/errors/NoInstanceFound.md for more information,
or to contribute content related to this error.

I understand error as ty is of unknown type, but I would like to awoid specify it to by able to work with everithing in typeclass ParseParam…

Thank you in advance

#2

The problem is that you haven’t said anything about the contents of the params record; presumably you want to assert that the record field identified by the label symbol has type ty? If so, I think you can do that with a Cons constraint, like Cons label ty unused params.

1 Like
#3

Thank you, that was it