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Extensible Record in Haskell using Vinyl for Noob

soriyeak profile image soriyeak ・3 min read

In this tutorial, I'm going to show how to use vinyl to create an extensible record within haskell.

This tutorials aim for those who area already familiar with haskell, but cannot figure out how to use vinyl.

We are going to use javascript object/typescript interface as comparison.

Note

  • We are using stack lts-17.1

Setting up the project

  • Using stack new learn-vinyl to create a new project.

  • Add these library to your package.yaml: vinyl, microlens

  • In Lib.hs, add this language extensions

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE NoMonomorphismRestriction #-}
{-# LANGUAGE OverloadedLabels #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-} 
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  • Add these imports:
import Data.Vinyl
import Data.Vinyl.Syntax ()
import Lens.Micro
import Data.Text
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Defining a Record

Let's define a record Person:

type Person = FieldRec
  '[ "name" ::: Text
   , "age" ::: Int
   , "is_single" ::: Bool
   ]
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this is equivalent to this typescript version:

type Person = {
  name: string,
  age: number,
  is_single: boolean
}
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Constructing the record

To construct a record of type Person, we do this:

marcus :: Person
marcus =
      #name =:= "Marcus"
  <+> #age =:= 58
  <+> #is_single =:= False
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Typescript version:

let marcus : Person = 
  {
    name: "Marcus", 
    age: 58, 
    is_single: false
  }
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Warning: vinyl record field takes order into the matter, so if you put the wrong order, it will causes the type error.

Example:

bad_record :: Person
bad_record =
      #age =:= 58
  <+> #name =:= "Marcus"
  <+> #is_single =:= False
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This is not good, luckily we have a function to re-order the field for us, by using rcast:

marcus2 :: Person
marcus2 = rcast $
      #age =:= (58 :: Int)
  <+> #name =:= ("Marcus" :: Text)
  <+> #is_single =:= False

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One bad thing is that, you have to explicitly add the type for some fields.

Accessing the field

To access the field of a record, we can use (^.) operators in microlens:

run :: IO ()
run = do
  print (marcus ^. #name)
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TS version:

console.log(marcus.name)
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Setting the field of a record

run :: IO ()
run = do
  let updated_marcus = marcus & #name .~ "Aurelius"
  print (updated_marcus ^. #name)
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Typescript version:

marcus.name = "Aurelius"
console.log(marcus)
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Nested Record

Constructing

To construct a nested record, we add this:

type Empire = FieldRec
  '[ "king" ::: Person
   , "country" ::: Text
   ]

rome :: Empire
rome = 
      #king =:= marcus
  <+> #country =:= "Italy"
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Accessing nested field

run :: IO ()
run = do
  print (rome ^. #king . #name)
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Typescript version:

console.log(rome.king.name)
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Setting nested field

run :: IO ()
run = do
  let updated_rome = rome & #king . #name .~ "Aurelius"
  print (updated_rome ^. #king . #name)
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rome.king.name = "Aurelius"
console.log(rome)
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Conclusion

vinyl is a very powerful library, yet it lags a simple tutorial. That is the reason why I create this tutorial.

In the future, we will talk about record subset, combining record, and more.

Full source code:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE NoMonomorphismRestriction #-}
{-# LANGUAGE OverloadedLabels #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}

module Lib
    ( run
    ) where

import Data.Vinyl
import Data.Vinyl.Syntax ()
import Lens.Micro
import Data.Text

type Person = FieldRec
  '[ "name" ::: Text
   , "age" ::: Int
   , "is_single" ::: Bool
   ]

type Empire = FieldRec
  '[ "king" ::: Person
   , "country" ::: Text
   ]

marcus :: Person
marcus =
      #name =:= "Marcus"
  <+> #age =:= 58
  <+> #is_single =:= False

-- bad_record :: Person
-- bad_record =
--       #age =:= 58
--   <+> #name =:= "Marcus"
--   <+> #is_single =:= False

marcus2 :: Person
marcus2 = rcast $
      #age =:= (58 :: Int)
  <+> #name =:= ("Marcus" :: Text)
  <+> #is_single =:= False

rome :: Empire
rome =
      #king =:= marcus
  <+> #country =:= "Italy"


run :: IO ()
run = do
  print (marcus ^. #name)

  let updated_marcus = marcus & #name .~ "Aurelius"
  print (updated_marcus ^. #name)

  -- Nested
  print (rome ^. #king . #name)

  let updated_rome = rome & #king . #name .~ "Aurelius"
  print (updated_rome ^. #king . #name)

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