• Eric Seidel's avatar
    Don't infer CallStacks · 7407a66d
    Eric Seidel authored
    We originally wanted CallStacks to be opt-in, but dealing with let
    binders complicated things, forcing us to infer CallStacks. It turns
    out that the inference is actually unnecessary though, we can let the
    wanted CallStacks bubble up to the outer context by refusing to
    quantify over them. Eventually they'll be solved from a given CallStack
    or defaulted to the empty CallStack if they reach the top.
    
    So this patch prevents GHC from quantifying over CallStacks, getting us
    back to the original plan. There's a small ugliness to do with
    PartialTypeSignatures, if the partial theta contains a CallStack
    constraint, we *do* want to quantify over the CallStack; the user asked
    us to!
    
    Note that this means that
    
      foo :: _ => CallStack
      foo = getCallStack callStack
    
    will be an *empty* CallStack, since we won't infer a CallStack for the
    hole in the theta. I think this is the right move though, since we want
    CallStacks to be opt-in. One can always write
    
      foo :: (HasCallStack, _) => CallStack
      foo = getCallStack callStack
    
    to get the CallStack and still have GHC infer the rest of the theta.
    
    Test Plan: ./validate
    
    Reviewers: goldfire, simonpj, austin, hvr, bgamari
    
    Reviewed By: simonpj, bgamari
    
    Subscribers: bitemyapp, thomie
    
    Projects: #ghc
    
    Differential Revision: https://phabricator.haskell.org/D1912
    
    GHC Trac Issues: #11573
    7407a66d
TcSimplify.hs 86.6 KB