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-vinylto create a new project.Add these library to your
package.yaml:vinyl,microlensIn
Lib.hs, add this language extensions
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE NoMonomorphismRestriction #-}
{-# LANGUAGE OverloadedLabels #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}
- Add these imports:
import Data.Vinyl
import Data.Vinyl.Syntax ()
import Lens.Micro
import Data.Text
Defining a Record
Let's define a record Person:
type Person = FieldRec
'[ "name" ::: Text
, "age" ::: Int
, "is_single" ::: Bool
]
this is equivalent to this typescript version:
type Person = {
name: string,
age: number,
is_single: boolean
}
Constructing the record
To construct a record of type Person, we do this:
marcus :: Person
marcus =
#name =:= "Marcus"
<+> #age =:= 58
<+> #is_single =:= False
Typescript version:
let marcus : Person =
{
name: "Marcus",
age: 58,
is_single: false
}
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
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
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)
TS version:
console.log(marcus.name)
Setting the field of a record
run :: IO ()
run = do
let updated_marcus = marcus & #name .~ "Aurelius"
print (updated_marcus ^. #name)
Typescript version:
marcus.name = "Aurelius"
console.log(marcus)
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"
Accessing nested field
run :: IO ()
run = do
print (rome ^. #king . #name)
Typescript version:
console.log(rome.king.name)
Setting nested field
run :: IO ()
run = do
let updated_rome = rome & #king . #name .~ "Aurelius"
print (updated_rome ^. #king . #name)
rome.king.name = "Aurelius"
console.log(rome)
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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