如何解開 Maybe Future ?

若先有 Function 回傳 Maybe，再根據 Maybe 內的 Value 呼叫 API，就會出現 Maybe Future。

Version

Sanctuary 3.1.0
Fluture 14.0.0

Maybe Future

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

let { pipe, map, Just, Nothing, add, I, maybe } = create ({ checkTypes: true, env: env.concat (flutureEnv) })

let data = Just (1)
// let data = Nothing

pipe ([
  map (add (1)),
  map (encase (add (2))),
  maybe (reject (0)) (I),
  fork (error) (log)
]) (data)

第 8 行

let data = Just (1)
// let data = Nothing

data 為 Maybe。

11 行

pipe ([
  map (add (1)),
  map (encase (add (2))),
  maybe (reject (0)) (I),
  fork (error) (log)
]) (data)

使用 pipe 組合 IIFE：

map (add (1))：將 Maybe 與一般 function 綁定，回傳 Maybe
map (encase (add (2)))：將 Maybe 與回傳 Future 的 function 綁定，回傳 Myabe Future
maybe (reject (0)) (I)：從外層開始依序解開 Maybe 與 Future，因此先使用 maybe 解開 Maybe，唯接下來會以 fork 解開 Future，因此 Nothing 必須以 reject (0) 表示，不能只是 0，Sanctuary 會抱怨 type 不對
fork (error) (log)：最後再解開 Future

maybe000

Conclusion

無論是 Future Maybe 或 Maybe Future，拆開的關鍵都是先解 Maybe，最後解 Future，永遠將 fork 放在 pipe 內最後一行