haskell-jp / questions #22

app\record-placeholder.hs:53:23: error:
    • Couldn't match type ‘LabelPhantom
                             k1
                             (Inextensible (Field Proxy) '[k1 ':> ()])
                             (Proxy (Inextensible (Field Proxy) '[k1 ':> ()]))’
                     with ‘Record xs -> Const a0 (Record xs)’
      Expected type: Control.Lens.Getter.Getting a0 (Record xs) a0
        Actual type: FieldName k1

data Free f a = Pure a | Free (f (Free f a)) deriving Functor

data Tree a = Node a Tree Tree | Leaf a

data T a = N a (T a) (T a) | L a

import Data.Type.Bool
import Data.Type.Equality

someFun :: (a == Foo || a == Bar) ~ True => ...

{-# LANGUAGE ConstraintKinds, TypeFamilies #-}
import Data.Constraint
data Foo
data Bar

type family IsFooOrBar x :: Constraint where
  IsFooOrBar Foo = ()
  IsFooOrBar Bar = ()
  IsFooOrBar x = Bottom

someFun :: (IsFooOrBar a, Monad m) => m a
someFun = undefined

{ "tickets":[],
  "next_page": some_url
}

{ "users":[],
  "next_page": some_url
}

{-# LANGUAGE TemplateHaskell #-}
module Foo where

import Language.Haskell.TH.Syntax

foo = $(lift ("foo" ++ "bar"))

{-# LANGUAGE TemplateHaskell #-}
module Foo where

import Language.Haskell.TH.Syntax

foo = "foobar"

xs = [1..] :: [Int]
let (x: _) = head $ drop 10 xs

type family Fst (t :: (a, b)) :: a where
  Fst '(a, b) = a

type family Snd (t :: (a, b)) :: b where
  Snd '(a, b) = b

extractedTuple :: '(Fst a, Snd a) :~: a