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copied!<p>This is a limitation, but one with good reason: if it did not work that way what would the expected semantics of </p> <pre><code>runState :: State t s a -> s -> (s,a) runState (State f) s = f s example :: s -> a example = snd $ runState ((State undefined) >> return 1) () </code></pre> <p>well, it could be</p> <pre><code>example = snd $ runState ((State undefined) >>= \_ -> return 1) () = snd $ runState (State $ \s -> case undefined s of (s',_) -> (s',1)) () = snd $ case undefined () of (s',_) -> (s',1) = snd $ case undefined of (s',_) -> (s',1) = snd undefined = undefined </code></pre> <p>or it could be</p> <pre><code>example = snd $ runState ((State undefined) >>= \_ -> return 1) () = snd $ runState (State $ \s -> case undefined s of ~(s',_) -> (s',1)) () = snd $ case undefined () of ~(s',_) -> (s',1) = snd $ (undefined,1) = 1 </code></pre> <p>these are not the same. One option would be to define a function an extra class, like</p> <pre><code>class IsStrictness t where bindState :: State t s a -> (a -> State t s b) -> State t s b </code></pre> <p>and then then define </p> <pre><code>instance IsStrictness t => Monad (State t s) where return = returnState (>>=) = bindState </code></pre> <p>and instead of defining <code>bindState</code> as part of <code>IsStrictness</code>, you can use a singleton</p> <pre><code>data SingStrictness (t :: Strictness) where SingStrict :: SingStrictness Strict SingLazy :: SingStrictness Lazy class IsStrictness t where singStrictness :: SingStrictness t bindState :: IsStrictness t => State t s a -> (a -> State t s b) -> State t s b bindState ma' amb' = go singStrictness ma' amb' where go :: SingStrictness t -> State t s a -> (a -> State t s b) -> State t s b go SingStrict ma amb = ... go SingLazy ma amb = ... </code></pre> <p>which can be enhanced even further using the singelton infrastructures from GHC 7.6 instead of the custom class and singleton type. You will end up with</p> <pre><code>instance SingI t => Monad (State t s) </code></pre> <p>which is really not so scary. Get used to having lots of <code>SingI _</code> in your constraint sets. This is how it is going to work for at least a while, and isn't that ugly.</p> <p>As to why <code>State t [Bool]</code> is not deducible from <code>State t s</code>: the problem is that <code>State t s</code> is in your top level context, which means that <code>s</code> is quantified at the outermost level. You are defining a function which says "for any t and s such that Monad (State t s) I will give you ...". But, this doesn't say "for any t such that Monad (State t [Bool]) I will give you ...". Sadly, these universally quantified constraints are not so easy in Haskell. </p>

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