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Comment by eutectic | Hacker News Reader

It is an equality assertion: https://downloads.haskell.org/~ghc/7.6.1/docs/html/users_gui...

Thanks, but I still don't get it. So SpockState m equals BlockChainState? So getState's type is m BlockChainState?

    (BlockChainState chain _ _) <- getState

But it has to be anyway so, I guess I'm still confused. Is the squiggle for extra documentation or is it a hint to help the type inference? Why would type inference need help? Does it have to do with some of the extensions that are applied in the source (but appear to be otherwise unused)?

Yeah, it's for the compiler, otherwise it can't figure out that a BlockChainState is actually a SpockState m. I'm not really sure how it could otherwise, since the definition is completely decoupled from Spock.

It should be noted that it's entirely possible there's a less confusing way to write that type signature. I'm far from an expert in Haskell.

I'm farther from an expert than you. I tried (and failed) to find a less confusing way...

One thing I see people doing when they have real gnarly type signature is to concentrate the horror in one place and paint a veneer of simplicity elsewhere:

    type Get stateType returnType
      = forall monad.
      (SpockState monad ~ stateType, MonadIO monad, HasSpock monad)
      => monad returnType

Then elsewhere:

    getBlockChain :: Get BlockChainState [Block]
    getLatestBlock :: Get BlockChainState Block

This may only help superficially as type errors will still be hard-to-understand.

Ah, I think this is a better though! If you got this working, feel free to open up a PR. Or I can look into it when I get time.

Also, unless I'm mistaken, TypeFamilies are the relevant GHC extension here. I'm not exactly sure how this would be written differently without that

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