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Comment by m-j-fox | Hacker News Reader

Nicely done. It reads like Ikea instructions. I didn't really know how blockchain worked until I read this code. If this was written as a teaching tool or a portfolio piece or even an art project, then I say its a big success.

One question:

    getBlockChain :: (SpockState m ~ BlockChainState, MonadIO m, HasSpock m) => m [Block]

What does the squiggle mean?

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If you learned a thing about blockchains from this project, I'd call it a big success too! :D So happy to hear it was helpful

Thanks for making this. It reminds me of the day I fell in love with Haskell. I wanted to learn how a Verilog simulator works. There's a couple good open-source ones but I found the code impenetrable, so I went looking for something like an introductory course and found this obscure, disused code called Hydra which is hard to find. Let me see ... here it is:

https://github.com/garry-sharp/x/tree/master/CA4/HydraLib-0....

After reading for just an hour, the lightbulb came on and I not-only felt I had learned how a digital simulator works but also what a mighty hammer Haskell can be for teaching.

Since then I've read (and unfortunately written) a lot of Haskell that is far less elucidating. It's great to come across something that reminds me how I got turned on in the first place and that I should aspire to produce code that not-only barely gets the job done but also explains itself and embodies a modicum of beauty. Thanks again.

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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