如何解開 Future Either Maybe ? | 點燈坊

Future Either Maybe 為常見的 Monad Stack，如呼叫 API 回傳 Future，再使用回傳 Either 的 Function 判斷 API 回傳值，又使用回傳 Maybe 的 Function 運算，就會出現 Future Either Maybe。

Version

Fluture 14.0.0
Sanctuary 3.1.0

Fluture Either Maybe

import { create, env } from 'sanctuary'
import { thunkify } from 'ramda'
import { resolve, reject, fork } from 'fluture'
import { env as flutureEnv } from 'fluture-sanctuary-types'
import { toMaybe, error, log } from 'wink-fp'

let { pipe, map, add, ifElse, equals, Left, Right, either, maybe, I } = create ({ checkTypes: true, env: env.concat (flutureEnv) })

// let data = resolve (1)
let data = resolve (2)
// let data = reject (1)

let chkOne = ifElse 
  (equals (1))
  (thunkify (Left) (`Can't input one`))
  (Right)

let add2 = map (toMaybe) (add (2)) 

pipe ([
  map (chkOne), 
  map (map (add2)), 
  map (map (maybe (0) (I))),
  map (either (I) (I)),
  fork (error) (log)
]) (data) 

第 9 行

// let data = resolve (1)
let data = resolve (2)
// let data = reject (1)

data 為 Future。

13 行

let chkOne = ifElse 
  (equals (1))
  (thunkify (Left) (`Can't input one`))
  (Right)

chkOne 回傳 Either，若輸入為 1 則回傳 Left，否則回傳 Right。

18 行

let add2 = map (toMaybe) (add (2))

add2 回傳 Maybe，使用 map 組合 add (2) 與 toMaybe。

20 行

pipe ([
  map (chkOne), 
  map (map (add2)), 
  map (map (maybe (0) (I))),
  map (either (I) (I)),
  fork (error) (log)
]) (data) 

使用 pipe 組合 IIFE：

• map (chkOne)：data 為 Future，chkOne 回傳 Either，最後為 Future Either
• map (map (add2))：add2 回傳 Maybe，最後為 Future Either Maybe
• map (map (maybe (0) (I)))：解開 Mabye
• map (either (I) (I))：解開 Either
• fork (error) (log)：解開 Future

Conclusion

• Future Either Maybe 看起來很嚇人，關鍵是從內往外解，先解開 Maybe，再解開 Either，最後解開 Future，永遠將 fork 放在 pipe 內最後一行