• Simon Peyton Jones's avatar
    Eliminate so-called "silent superclass parameters" · a6f0f5ab
    Simon Peyton Jones authored
    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
CmmNode.hs 29.9 KB