#7862: Could not deduce (A) from the context (A, ...) -------------------------------------+------------------------------------- Reporter: alang9 | Owner: Type: bug | Status: closed Priority: normal | Milestone: 8.0.1 Component: Compiler (Type | Version: 7.8.3 checker) | Resolution: fixed | Keywords: Operating System: Linux | Architecture: x86_64 | (amd64) Type of failure: GHC rejects | Test Case: indexed- valid program | types/should_compile/T7862 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by thomie): * status: new => closed * resolution: => fixed * milestone: => 8.0.1 Comment: This was fixed in commit a6f0f5ab45b2643b561e0a0a54a4f14745ab2152: {{{ Author: Simon Peyton Jones <simonpj@microsoft.com> Date: Tue Dec 23 15:39:50 2014 +0000 Eliminate so-called "silent superclass parameters" The purpose of silent superclass parameters was to solve the awkward problem of superclass dictinaries being bound to bottom. See THE PROBLEM in Note [Recursive superclasses] in TcInstDcls Although the silent-superclass idea worked, * It had non-local consequences, and had effects even in Haddock, where we had to discard silent parameters before displaying instance declarations * It had unexpected peformance costs, shown up by Trac #3064 and its test case. In monad-transformer code, when constructing a Monad dictionary you had to pass an Applicative dictionary; and to construct that you neede a Functor dictionary. Yet these extra dictionaries were often never used. (All this got much worse when we added Applicative as a superclass of Monad.) Test T3064 compiled *far* faster after silent superclasses were eliminated. * It introduced new bugs. For example SilentParametersOverlapping, T5051, and T7862, all failed to compile because of instance overlap directly because of the silent-superclass trick. So this patch takes a new approach, which I worked out with Dimitrios in the closing hours before Christmas. It is described in detail in THE PROBLEM in Note [Recursive superclasses] in TcInstDcls. Seems to work great! Quite a bit of knock-on effect * The main implementation work is in tcSuperClasses in TcInstDcls Everything else is fall-out * IdInfo.DFunId no longer needs its n-silent argument * Ditto IDFunId in IfaceSyn * Hence interface file format changes * Now that DFunIds do not have silent superclass parameters, printing out instance declarations is simpler. There is tiny knock-on effect in Haddock, so that submodule is updated * I realised that when computing the "size of a dictionary type" in TcValidity.sizePred, we should be rather conservative about type functions, which can arbitrarily increase the size of a type. Hence the new datatype TypeSize, which has a TSBig constructor for "arbitrarily big". * instDFunType moves from TcSMonad to Inst * Interestingly, CmmNode and CmmExpr both now need a non-silent (Ord r) in a couple of instance declarations. These were previously silent but must now be explicit. * Quite a bit of wibbling in error messages }}} The test output is now: {{{ T7862.hs:23:10: Warning: No explicit implementation for ‘+’, ‘*’, ‘abs’, ‘signum’, ‘fromInteger’, and (either ‘negate’ or ‘-’) In the instance declaration for ‘Num (Tower s a)’ }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7862#comment:10> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler