Questions tagged [undecidable-instances]
12 questions
28
votes
2 answers
Why does this code using UndecidableInstances compile, then generate a runtime infinite loop?
When writing some code using UndecidableInstances earlier, I ran into something that I found very odd. I managed to unintentionally create some code that typechecks when I believed it should not:
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE…
Alexis King
- 43,109
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7
votes
3 answers
How can undecidable instances actually hang the compiler?
By the time I first read serious criticism on -XUndecidableInstances, I had already completely accustomed to it, seeing it as merely removal of an annoying restriction Haskell98 has to make compilers easier to implement.
In fact I've encountered…
leftaroundabout
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2
votes
1 answer
Is it able to avoid UndecidableInstances in this example?
I am trying to implement binary tree as a typeclass:
class BinaryTree bt where
empty :: bt a -> Bool
empty = isNothing . root
emptyTree :: bt a
branch :: a -> bt a -> bt a -> bt a
root :: bt a -> Maybe a
lbranch :: bt a ->…
qimokao
- 93
- 5
2
votes
2 answers
Transitive 'Subset` class for type-level-sets
I'm working with Dominic Orchard's type-level-sets library, which follows his paper pretty closely.
I'm building a DSL for expressing communication between parties during a synchronous concurrent program. One thing I'm going to need is the ability…
ShapeOfMatter
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2
votes
1 answer
GHC stuck due to UndecidableSuperClasses - expected behaviour or bug?
The following snippet makes GHC (checked with 8.6.2 & 8.4.4) stuck during compilation:
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE UndecidableSuperClasses #-}
import GHC.Exts…
yairchu
- 23,680
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2
votes
2 answers
Can I automatically produce typeclass instances for a conversion function without being overly permissive?
Recently, I asked a question about an instance I had created generating a runtime infinite loop, and I got a wonderful answer! Now that I understand what was going on, I have a new question: can I fix my attempt to accomplish my original goal?
Let…
Alexis King
- 43,109
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1
vote
0 answers
Example of the undecidability of higher-order unification
Could you demonstrate the undecidability of higher-order unification by one example?
For example, please show by one example that unification in lambda-calculus is undecidable.
nikkkk
- 11
- 1
1
vote
0 answers
Using Barbies' AllBF in an instance head without UndecidableInstances
I'd like to use AllBF from barbies in an instance head, like so:
import Barbies
import Barbies.Constraints
class MyClass a where
instance (ConstraintsB b, AllBF MyClass f b) => MyClass (Barbie b f) where
This fails because of a hidden…
Cactus
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1
vote
1 answer
Haskell typeclass constraint cannot be resolved due to Paterson's conditions
I'm trying to build an AST with indexed nested annotations. I added a typeclass for peeling the annotation at the top-level and tried to provide default instances that effectively says "if you know how to peel an annotation on its own, then you know…
madgen
- 747
- 3
- 15
1
vote
1 answer
Turing machine decidability ambiguous cases
1) Is a Turing machine M that accepts the language L = {ε}, accepting no entry?
In one hand, I think it can be false because the empty word could be an entry, but in another i think this could possibly be an indecidable problem.
2) Is every Turing…
Cmôn
- 23
- 2
1
vote
0 answers
Writing generic Monoid over Cofree; undecidable?
I'm trying to write the following Monoid instance for Cofree:
instance (Monoid a, Monoid (f (Cofree f a))) => Monoid (Cofree f a) where
mempty = mempty :< mempty
(a :< rest) `mappend` (b :< rest') = (a `mappend` b) :< (rest `mappend` rest')
but…
Chris Penner
- 1,881
- 11
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0
votes
0 answers
Turing Machine for regular languages
Theorem 5.3 from Sipser's TOC book is about decidability of Regular_TM = {M | M is a Turing Machines (TMs) and L(M) is regular languages}. For the sake of reaching a contradiction, TM R is assumed to be a decider for Regular_TM and then R is used to…
Mba7eth
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