Sanctuary 初體驗 | 點燈坊

Sanctuary 為著名的 Functional Library，其比 Ramda 嚴格，也能在 Runtime 檢查型別，且完整支援 Fantasy Land 的 Algebraic Data Type，寫起來更有 Haskell 感覺。

Version

Sanctuary 3.1.0

Maybe

import { propEq } from 'ramda'
import { pipe, find } from 'sanctuary'

let data = [
  { title: 'FP in JavaScript', price: 100 },
  { title: 'Elm in Action', price: 200 },
  { title: 'Speaking JavaScript', price: 300 }
]

let f = s => pipe ([
  find (propEq ('title') (s))
])

f ('Speaking JavaScript') (data) // ?
f ('Elm') (data) // ?

第 2 行

import { pipe, find } from 'sanctuary'

不是使用 Ramda 的 find，而是使用 Sanctuary 的 find，因為回傳 Maybe。

第 10 行

let f = s => pipe ([
  find (propEq ('title') (s))
])

使用 pipe 組合 f：

• find (propEq ('title') (s))：尋找 title 為 s 的 Maybe

find
Foldable f => (a -> Boolean) -> f a -> Maybe a
回傳第一個符合 predicate 的 element，但回傳Maybe

a -> Boolean：判斷條件的 predicate

f a：data 為 Foldable

Maybe a：將結果包進 Maybe 內回傳

若找得到資料則回傳 Just，若找不到則回傳 Nothing。

fromMaybe()

import { propEq, pluck } from 'ramda'
import { pipe, find, fromMaybe } from 'sanctuary'

let data = [
  { title: 'FP in JavaScript', price: 100 },
  { title: 'Elm in Action', price: 200 },
  { title: 'Speaking JavaScript', price: 300 }
]

let f = s => pipe ([
  find (propEq ('title') (s)),
  pluck ('price'),
  fromMaybe (0)
])

f ('Speaking JavaScript') (data) // ?
f ('Elm') (data) // ?

10 行

let f = s => pipe ([
  find (propEq ('title') (s)),
  pluck ('price'),
  fromMaybe (0)
])

使用 pipe 組合 f：

• find (propEq ('title') (s))：尋找 title 為 s 的 Maybe
• pluck ('price')：在 Maybe 內取得 price
• fromMaybe(0)：若為 Nothing 則回傳 0

fromMaybe
a -> Maybe a -> a
從 Maybe 取出內部值，並提供 Nothing 預設值

a：Nothing 的預設值

Maybe a：data 為 Maybe

a：回傳 Maybe 內部值

Conclusion

• 理論上 Ramda 的 function 若可能回傳 undefined，則建議改用 Sanctuary 的 function 改回傳 Maybe

Reference

Sanctuary, find
Sanctuary, fromMaybe