#3660: "Var.tcTyVarDetails" exception w/ Assoc. Datatypes and Monad Transformers
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Reporter: jfredett | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.10.4 | Severity: minor
Keywords: Monad Transformers, Associated Datatypes | Testcase:
Os: Linux | Architecture: x86
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Summary:
When trying to build a monad stack which involves a state transformer with
a constrained state, the newtype which constrains said state (below, it is
the `Filter` newtype) cannot seem to derive something within the list of
things it's intended to derive. After some testing I found that changing
the `MonadState Bool` part of the `deriving` clause to `MonadState t` (in
line with the actual state parameter) made everything work. The Type of
`Email` doesn't matter, I've tested to this effect (replacing `Email` with
`data Email = Email` and not importing the real version, it still compiles
fine in the fixed case, and does not compile in the broken case).
I have not tested on GHC 6.12 yet, I'm still trying to install it. All
other system information at the bottom.
By the looks of it, this is a case of GHC not noticing I'm doing something
silly, and not reporting something along the lines of the "This isn't
polymorphic enough" error.
Broken Code:
{{{
type Context = ReaderT Email
type Match t = StateT t IO
type ContextMatch t a = Context (Match t) a
newtype FilterState t => Filter t a = Filter (ContextMatch t a)
deriving (Functor, Monad, MonadReader Email, MonadState Bool,
MonadIO)
class FilterState t where
data FState t
deliver :: FState t -> IO ()
}}}
Error:
{{{
[1 of 3] Compiling Network.HackMail.Email.ParseEmail (
Network/HackMail/Email/ParseEmail.hs, interpreted )
[2 of 3] Compiling Network.HackMail.Email.Email (
Network/HackMail/Email/Email.hs, interpreted )
[3 of 3] Compiling Network.HackMail.Filter.Filter (
Network/HackMail/Filter/Filter.hs, interpreted )
*** Exception: No match in record selector Var.tcTyVarDetails
}}}
Fixed Code:
{{{
type Context = ReaderT Email
type Match t = StateT t IO
type ContextMatch t a = Context (Match t) a
-- changed `Bool` to `t`.
newtype FilterState t => Filter t a = Filter (ContextMatch t a)
deriving (Functor, Monad, MonadReader Email, MonadState t, MonadIO)
class FilterState t where
data FState t
deliver :: FState t -> IO ()
}}}
System Info:
{{{
[jfred...@erdos]$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.10.4
[jfred...@erdos]$ uname -a
Linux Erdos 2.6.31-ARCH #1 SMP PREEMPT Fri Oct 23 11:12:58 CEST 2009 i686
Intel(R) Celeron(R) CPU 3.06GHz GenuineIntel GNU/Linux
}}}
(Possibly) Related Tickets include: 3621, 3422 and 2714
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3660>
GHC <http://www.haskell.org/ghc/>
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