Hi, Am Mittwoch, den 20.01.2016, 00:39 -0500 schrieb Richard Eisenberg:
I'm sure there's an easy answer to this, but I'm wondering: why is the CallStack feature implemented with implicit parameters instead of just a magical constraint? Whenever I use this feature, I don't want to have to enable -XImplicitParams and then make sure I get the name right. What would be wrong with, e.g.,
undefined :: AppendsCallStack => a
Seems simpler. Is it problems with a nullary class?
I tried to construct this using what we have right now: ==> AppendCallStack.hs <== {-# LANGUAGE ConstraintKinds, ImplicitParams #-} module AppendCallStack (AppendsCallStack) where import GHC.Stack type AppendsCallStack = ?x::CallStack ==> Bar.hs <== module Bar where import AppendCallStack foo x :: AppendsCallStack => a -> a foo x = error "Test" But with GHC-7.10 I get [1 of 2] Compiling AppendCallStack ( AppendCallStack.hs, AppendCallStack.o ) [2 of 2] Compiling Bar ( Bar.hs, Bar.o ) Bar.hs:5:1: Invalid type signature: foo x :: AppendsCallStack => a -> a Should be of form <variable> :: <type> although the constraint seems to be a constraint all right: *AppendCallStack> :kind AppendsCallStack AppendsCallStack :: GHC.Prim.Constraint and with GHC HEAD I get [1 of 2] Compiling AppendCallStack ( AppendCallStack.hs, AppendCallStack.o ) AppendCallStack.hs:6:1: error: • Illegal implicit parameter ‘?x::CallStack’ • In the type synonym declaration for ‘AppendsCallStack’ Too bad... Greetings, Joachim -- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0xF0FBF51F Debian Developer: nomeata@debian.org