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Questions tagged [monadfix]

22 questions
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3 answers

Understanding the sliding law of MonadFix

I intuitively understand the purity, tightening, and nesting laws of MonadFix. However, I'm having a hard time understanding the sliding law. mfix (fmap h . f) = fmap h (mfix (f . h)) -- for strict h My first question is that if h has to be strict…
Aadit M Shah
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Haskell: monadic fixpoint on RWS is looping if traversing on argument

I am writing a program which involves RWS for tracking mutable state and producing some log. My purpose is to define a computation that evaluates some action, gathers the aftercoming state and depending on it appends something to the beginning of…
radrow
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Mutually recursive evaluator in Haskell

Update: I've added an answer that describes my final solution (hint: the single Expr data type wasn't sufficient). I'm writing an evaluator for a little expression language, but I'm stuck on the LetRec construct. This is the language: type Var =…
Tom Lokhorst
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Why is `mfix` not total in `MaybeT`

The transformers implementation of MonadFix for MaybeT fails if the function ever evaluates to Nothing. Why is Nothing not propagating over mfix? mfix' :: MonadFix m => (a -> MaybeT m a) -> MaybeT m a mfix' f = MaybeT $ mfix $ \case Nothing ->…
fakedrake
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Duplicate EDSL code generated with RecursiveDo in Haskell

Using ghc v8.0.1, compiled with -O2 flag. I'm having a problem with RecursiveDo (mdo). There are two slightly different functions that should produce the same output, but they don't. The following function produces the correct output: proc2 ::…
Glaukon
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How to implement MonadFix instance of an IO-like type in JS?

I am completely lost in trying to translate the following Haskell code into Javascript: instance MonadFix IO where mfix f = do var <- newEmptyMVar -- 1 ans <- unsafeInterleaveIO $ takeMVar var -- 2 result <- f…
user5536315
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What is the meaning of tilde in lambda expression?

On the 7th page of this book there is a tilde (shown by red circle in the picture below) in the definition of toggle. What does the tilde mean ?
jhegedus
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