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haskellconcurrencypromisemonads
luochen1990
luochen1990

Reputation: 3867

Why `Concurrently` is not a monad in Haskell?

I'm reading the doc of package async, and trying to find something similar to JavaScript's Promise, and I find Concurrently, which is the most close concept that implemented Functor, Applicative (Promise.all), Alternative (Promise.race). But it doesn't implement Monad (Promise.then), I'm wondering why.

I think it maybe because (>>=) is a sequential operation, which conflict with the name Concurrently, but is this the only reason? is there some more important reason here?

Upvotes: 4

Views: 432

Answers (1)

Joseph Sible-Reinstate Monica
Joseph Sible-Reinstate Monica

Reputation: 48672

The Monad typeclass states that (<*>) and ap should be equivalent. (<*>) for Concurrently evaluates both the LHS and the RHS at the same time. ap can't evaluate the RHS until the LHS is done, since (>>=) needs to finish evaluating the LHS before it can call the function that gives it the RHS.

Upvotes: 5

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