#13320: Unfortunate compiler loop when creating type loop (with UndecidableInstances) -------------------------------------+------------------------------------- Reporter: Ptival | Owner: (none) Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: | UndecidableInstances loop Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by Ptival: Old description:

I'm afraid this will simply be seen as "that's what happens when you use UndecidableInstances", but I might as well document this issue I had.

Trying to play with a "Trees that Grow" syntax, I encountered an issue when making a mistake, that can be boiled down to the following:

{{{#!hs {-# language ConstraintKinds, FlexibleContexts, TypeFamilies, UndecidableInstances #-}

module Loop where

import GHC.Exts (Constraint) import Test.QuickCheck

type family X_Var ξ

data TermX ξ = Var (X_Var ξ)

type ForallX (φ :: * -> Constraint) ξ = ( φ (X_Var ξ) )

--genTerm :: ForallX Arbitrary ξ => Int -> Gen (TermX ξ) genTerm 0 = Var <$> arbitrary genTerm n = Var <$> genTerm (n - 1)

--instance ForallX Arbitrary ξ => Arbitrary (TermX ξ) where --arbitrary = sized genTerm

}}}

This code will compile correctly, and generate:

{{{#!hs genTerm :: (X_Var ξ ~ TermX ξ, Arbitrary (TermX ξ), Eq t, Num t) => t -> Gen (TermX ξ) }}}

Which is correct (though, not the type I had intended, since my code had a mistake).

Now, if you uncomment the "instance" line only, the compiler will loop.

Adding the commented out type, of course, gives a type error where it's due.

I was just wondering whether this type of error failed with the loops that should be caught by the compiler.

New description: I'm afraid this will simply be seen as "that's what happens when you use UndecidableInstances", but I might as well document this issue I had. Trying to play with a "Trees that Grow" syntax, I encountered an issue when making a mistake, that can be boiled down to the following: {{{#!hs {-# language ConstraintKinds, FlexibleContexts, TypeFamilies, UndecidableInstances #-} module Loop where import GHC.Exts (Constraint) import Test.QuickCheck type family X_Var ξ data TermX ξ = Var (X_Var ξ) type ForallX (φ :: * -> Constraint) ξ = ( φ (X_Var ξ) ) --genTerm :: ForallX Arbitrary ξ => Int -> Gen (TermX ξ) genTerm 0 = Var <$> arbitrary genTerm n = Var <$> genTerm (n - 1) --instance ForallX Arbitrary ξ => Arbitrary (TermX ξ) where --arbitrary = sized genTerm }}} This code will compile correctly, and generate: {{{#!hs genTerm :: (X_Var ξ ~ TermX ξ, Arbitrary (TermX ξ), Eq t, Num t) => t -> Gen (TermX ξ) }}} Which is correct (though, not the type I had intended, since my code had a mistake). Now, if you uncomment the "instance" line only, the compiler will loop. Adding the commented out type, of course, gives a type error where it's due. I was just wondering whether this type of error fell within the "loops that should be caught by the compiler". -- -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13320#comment:2> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#13320: Unfortunate compiler loop when creating type loop (with UndecidableInstances) -------------------------------------+------------------------------------- Reporter: Ptival | Owner: dfeuer Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: | UndecidableInstances loop Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Ah, commit 859dc65369e8a9722514046246dd32b683c8b4a9 (Yet another attempt at inferring the right quantification) was what fixed this. Hooray! -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13320#comment:4> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#13320: Unfortunate compiler loop when creating type loop (with UndecidableInstances) -------------------------------------+------------------------------------- Reporter: Ptival | Owner: dfeuer Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: | UndecidableInstances loop Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by David Feuer <David.Feuer@…>): In [changeset:"cb850e01560adf12e83fcf85f479636be17d017c/ghc" cb850e0/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="cb850e01560adf12e83fcf85f479636be17d017c" Add test for #13320 Reviewers: austin, bgamari Reviewed By: bgamari Subscribers: rwbarton, thomie GHC Trac Issues: #13320 Differential Revision: https://phabricator.haskell.org/D3532 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13320#comment:6> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#13320: Unfortunate compiler loop when creating type loop (with UndecidableInstances) -------------------------------------+------------------------------------- Reporter: Ptival | Owner: dfeuer Type: bug | Status: closed Priority: low | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.0.1 checker) | Keywords: Resolution: fixed | UndecidableInstances loop Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): phab:D3532 Wiki Page: | -------------------------------------+------------------------------------- Comment (by bgamari): Test merged with 561553fe424e2f2e3500b635655fe6d9c294c666. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13320#comment:8> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler