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Prove promise as a monad
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# Promise = require 'promise' | |
## | |
compare2Promise = (title, pA, pB) -> | |
valueA = null | |
valueB = null | |
pA.then (value) -> | |
valueA = value | |
pB.then (value) -> | |
valueB = value | |
Promise.all [pA, pB] | |
.then -> | |
console.log "==> #{title}" | |
console.log "value of A: #{valueA}" | |
console.log "value of B: #{valueB}" | |
console.log "equals is #{if valueA is valueB then 'true' else 'false'}" | |
### | |
The first law | |
M(mResult(x), mf) = mf(x) | |
### | |
promiseMonad = (mv, mf) -> | |
Promise::then.call mv, mf | |
x = "monad example" | |
# mResult is a function as Promise.resolve | |
promiseMonad.mResult = Promise.resolve.bind(Promise) | |
mf = (value) -> | |
new Promise (resolved, rejected) -> | |
resolved value + " monadF" | |
result1 = promiseMonad promiseMonad.mResult(x), mf | |
result2 = mf x | |
compare2Promise "First Law:", result1, result2 | |
### | |
The second law | |
M(mv, mResult) = mv | |
### | |
mv = Promise.resolve [4] | |
result1 = promiseMonad mv, promiseMonad.mResult | |
result2 = mv | |
compare2Promise "Second Law:", result1, result2 | |
### | |
The third law | |
M(M(mv, mf), mg)) = M(mv, function(x){return M(mf(x), mg)}) | |
### | |
mg = (value) -> | |
new Promise (resolved, rejected) -> | |
resolved value + " monadG" | |
result1 = promiseMonad promiseMonad(mv, mf), mg | |
result2 = Promise::then.call mv, (x) -> | |
Promise::then.call (mf x), mg | |
compare2Promise "Third Law:", result1, result2 | |
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According to Functional JavaScript(http://functionaljavascript.blogspot.tw/2013/07/monads.html) posted by Santosh Rajan, I write codes to prove Promise as a kind of monad.