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Today's Topics:
1. Putrady 2018 Ch. 3 - STM (Site Administrator)
2. Re: Putrady 2018 Ch. 3 - STM (Sylvain Henry)
----------------------------------------------------------------------
Message: 1
Date: Sun, 27 Oct 2019 12:58:44 -0600
From: Site Administrator <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Putrady 2018 Ch. 3 - STM
Message-ID: <[email protected]>
Content-Type: text/plain; charset="utf-8"; Format="flowed"
Hello,
I am new not only to Haskell, but to programming in general. I hope that
I am posting in the correct forum.
I am working through "Practical Web Development With Haskell" by Ecky
Putrady (2018), New York: Apress, and I am having some difficulty
understanding the STM portion of Chapter three. More specifically, I
cannot get the following code snippet to return the expected result in
the REPL:
-- snippet from Adapter.InMemory.Auth module as composed on pages
53-64 in the book
import ClassyPrelude
import qualified Domain.Auth as D -- as composed on pages 35-52 in
the book
import Conrol.Monad.Except
import Text.StringRandom
import Data.Has
-- ...
data State = State
{ stateAuths :: [(D.UserId, D.Auth)]
-- ...
initialState :: State
initialState = State
{ stateAuths = []
-- ...
type InMemory r m = (Has (TVar State) r, MonadReader r m, MonadIO m)
addAuth :: InMemory r m
=> D.Auth -> m (Either D.RegistrationError
D.VerificationCode)
addAuth auth = do
tvar <- asks getter
-- ... rest of function
These are the instructions for "Verification in REPL" provided on page
64 of the book:
> :l Adapter.InMemory.Auth
> let email = D.mkEmail "[email protected]"
> let passw = D.mkPassword "1234ABCDefgh"
> let auth = either undefined id $ D.Auth <$> email <*> passw
> s <- newTVarIO initialState
> addAuth s auth
Calling the addAuth function is supposed to print the following:
Right "aBNhtG653Bga9kas" -- (or whatever random vCode stringRandomIO
generates)
However, GHCI instead tells me:
<interactive>:6.9: error:
* Couldn't match expected type 'D.Auth'
with actual type 'TVar State'
* In the first argument of 'addAuth', namely 's'
In the expression: addAuth s auth
-- ...
The source code for the book at
https://github.com/Apress/practical-webdev-haskell was created with
stack resolver lts-9.11, while I am using resolver lts-14.11. I am doing
this intentionally, because debugging the compilation errors helps guide
me to become more familiar with the documentation in the packages being
implemented, as well as helping me to understand general Haskell
concepts little by little. I have tried looking through the changelogs
for the stm and classy-prelude packages on https://hackage.haskell.org,
for any clues which would be intelligible to me (an extremely
restrictive constraint, admittedly), that would help me understand why
the addAuth function thinks the TVar State we pass to it should be a
D.Auth, and why this presumably would not have been the case had I built
the project with the lts-9.11 resolver, but I have not been able to
understand it thus far.
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Message: 2
Date: Mon, 28 Oct 2019 09:46:10 +0100
From: Sylvain Henry <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Putrady 2018 Ch. 3 - STM
Message-ID: <[email protected]>
Content-Type: text/plain; charset="utf-8"; Format="flowed"
Hi,
The issue has nothing to do with STM.
> that would help me understand why the addAuth function thinks the
TVar State we pass to it should be a D.Auth
Let's look at the type of `addAuth`:
addAuth :: InMemory r m => D.Auth -> m (Either D.RegistrationError
D.VerificationCode)
It takes a single parameter with type `D.Auth`. So you don't have to
pass `s` when you call it.
The `TVar State` value is passed implicitly thanks to the constraint
`InMemory r m` (i.e. a value with type `r` can be obtained from `m`
Monad thanks to `MonadReader r m` and this value contains a `TVar State`
thanks to the `Has (TVar State) r` constraint`).
You can't call `addAuth` directly in GHCI (where `m` would be `IO`), you
have to follow the example here:
https://github.com/Apress/practical-webdev-haskell/blob/master/03/src/Lib.hs#L30
Or alternatively you can modify `addAuth` for you GHCI test:
addAuth :: TVar State -> D.Auth -> m (Etiher ...)
addAuth tvar auth = do
-- ... rest of function
Hope this helps,
Sylvain
On 27/10/2019 19:58, Site Administrator wrote:
> Hello,
>
> I am new not only to Haskell, but to programming in general. I hope
> that I am posting in the correct forum.
>
> I am working through "Practical Web Development With Haskell" by Ecky
> Putrady (2018), New York: Apress, and I am having some difficulty
> understanding the STM portion of Chapter three. More specifically, I
> cannot get the following code snippet to return the expected result in
> the REPL:
>
> -- snippet from Adapter.InMemory.Auth module as composed on pages
> 53-64 in the book
> import ClassyPrelude
> import qualified Domain.Auth as D -- as composed on pages 35-52 in
> the book
> import Conrol.Monad.Except
> import Text.StringRandom
> import Data.Has
> -- ...
> data State = State
> { stateAuths :: [(D.UserId, D.Auth)]
> -- ...
> initialState :: State
> initialState = State
> { stateAuths = []
> -- ...
> type InMemory r m = (Has (TVar State) r, MonadReader r m, MonadIO m)
>
> addAuth :: InMemory r m
> => D.Auth -> m (Either D.RegistrationError
> D.VerificationCode)
> addAuth auth = do
> tvar <- asks getter
> -- ... rest of function
>
> These are the instructions for "Verification in REPL" provided on page
> 64 of the book:
>
> > :l Adapter.InMemory.Auth
> > let email = D.mkEmail "[email protected]"
> > let passw = D.mkPassword "1234ABCDefgh"
> > let auth = either undefined id $ D.Auth <$> email <*> passw
> > s <- newTVarIO initialState
> > addAuth s auth
>
> Calling the addAuth function is supposed to print the following:
>
> Right "aBNhtG653Bga9kas" -- (or whatever random vCode
> stringRandomIO generates)
>
> However, GHCI instead tells me:
>
> <interactive>:6.9: error:
> * Couldn't match expected type 'D.Auth'
> with actual type 'TVar State'
> * In the first argument of 'addAuth', namely 's'
> In the expression: addAuth s auth
> -- ...
>
> The source code for the book at
> https://github.com/Apress/practical-webdev-haskell was created with
> stack resolver lts-9.11, while I am using resolver lts-14.11. I am
> doing this intentionally, because debugging the compilation errors
> helps guide me to become more familiar with the documentation in the
> packages being implemented, as well as helping me to understand
> general Haskell concepts little by little. I have tried looking
> through the changelogs for the stm and classy-prelude packages on
> https://hackage.haskell.org, for any clues which would be intelligible
> to me (an extremely restrictive constraint, admittedly), that would
> help me understand why the addAuth function thinks the TVar State we
> pass to it should be a D.Auth, and why this presumably would not have
> been the case had I built the project with the lts-9.11 resolver, but
> I have not been able to understand it thus far.
>
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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