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PODefining PrimMonad instance for STT? (ST Transformer)
 

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    <p><code>Data.Vector.Mutable</code> seems to require an instance of <code>PrimMonad</code> in <code>ST s</code> and <code>IO</code> monads.</p> <p>The typeclass is defined as this --</p> <pre><code>-- | Class of primitive state-transformer monads class Monad m =&gt; PrimMonad m where -- | State token type type PrimState m -- | Execute a primitive operation primitive :: (State# (PrimState m) -&gt; (# State# (PrimState m), a #)) -&gt; m a -- | Expose the internal structure of the monad internal :: m a -&gt; State# (PrimState m) -&gt; (# State# (PrimState m), a #) </code></pre> <p>They're implemented like this --</p> <pre><code>instance PrimMonad IO where type PrimState IO = RealWorld primitive = IO internal (IO p) = p instance PrimMonad (ST s) where type PrimState (ST s) = s primitive = ST internal (ST p) = p </code></pre> <p>I don't really understand at all what any of the functions of the typeclass are supposed to do, or how the implementations work.</p> <p>But I need to implement it for STT (the one given by <a href="http://hackage.haskell.org/package/STMonadTrans-0.3.1" rel="nofollow">http://hackage.haskell.org/package/STMonadTrans-0.3.1</a>)</p> <p><code>STT</code> has constructor <code>STT s m a</code></p> <p>In my naive attempt I tried replacing everything <code>ST s</code> with <code>STT s m</code>:</p> <pre><code>instance Monad m =&gt; PrimMonad (STT s m) where type PrimState (STT s m) = s primitive = STT internal (STT p m) = p </code></pre> <p>but I get this error:</p> <pre><code>Not in scope: data constructor `STT' </code></pre> <p>for the definitions of <code>primitive</code> and <code>internal</code>, despite having used <code>STT</code> multiple times throughout the program already (although I guess as a type constructor?).</p> <p>How should I properly implement this typeclass?</p> <p>(I will eventually be using this as <code>STT s (Rand g) a</code>)</p> <hr> <p>EDIT: I imported <code>Control.Monad.ST.Trans.Internal</code> to get <code>STT</code> as a data constructor, and these are the new errors: (after changing <code>internal (STT s m)</code> to <code>internal (STT s)</code>)</p> <pre><code>Couldn't match kind `*' against `ArgKind' Kind incompatibility when matching types: m0 :: * -&gt; * (#,#) (ghc-prim:GHC.Prim.State# (PrimState (STT s m))) :: ArgKind -&gt; (#) In the expression: STT In an equation for `primitive': primitive = STT Couldn't match type `m' with `(#,#) (ghc-prim:GHC.Prim.State# (PrimState (STT s m)))' `m' is a rigid type variable bound by the instance declaration at src/pimc/PIMC.hs:41:16 Expected type: ghc-prim:GHC.Prim.State# (PrimState (STT s m)) -&gt; (# ghc-prim:GHC.Prim.State# (PrimState (STT s m)), a #) Actual type: ghc-prim:GHC.Prim.State# s -&gt; m (STTRet s a) In the expression: p In an equation for `internal': internal (STT p) = p Couldn't match type `a' with `STTRet s a' `a' is a rigid type variable bound by the type signature for internal :: STT s m a -&gt; ghc-prim:GHC.Prim.State# (PrimState (STT s m)) -&gt; (# ghc-prim:GHC.Prim.State# (PrimState (STT s m)), a #) at src/pimc/PIMC.hs:44:3 Expected type: ghc-prim:GHC.Prim.State# (PrimState (STT s m)) -&gt; (# ghc-prim:GHC.Prim.State# (PrimState (STT s m)), a #) Actual type: ghc-prim:GHC.Prim.State# s -&gt; m (STTRet s a) In the expression: p In an equation for `internal': internal (STT p) = p </code></pre>
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    1. COYou'd need to import [the internal module](http://hackage.haskell.org/packages/archive/STMonadTrans/0.3.1/doc/html/Control-Monad-ST-Trans-Internal.html) to get access to the value constructor. But the wrapped thing, `(State# s -> m (STTRet s a))` has the wrong type. For `primitive`/`internal`, you need something that returns a tuple of state and result. For some monads `m`, you could write an instance, but you can't write a general instance, as far as I can see.
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      1. USDaniel Fischer
    2. CO@DanielFischer imported; but i run into the errors you mention. Any hint on how to make it work for a specific monad?
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    3. COI don't think there's a way to solve your problem. Defining the `primitive` function is no problem but the `internal` function is problematic even for simple monads such as the writer monad or the reader monad. I haven't been able to come up with a solution anyway. I think you would have to generalize PrimMonad somehow in order to achieve what you want to do.
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