Hi, Am Mittwoch, den 20.01.2016, 10:32 +0000 schrieb Simon Peyton Jones:
foo x :: AppendsCallStack => a -> a
Remove the "x"!
heh. Silly me. So let’s try again: With 7.10 it now works: ==> AppendCallStack.hs <== {-# LANGUAGE ConstraintKinds, ImplicitParams #-} module AppendCallStack (AppendsCallStack) where import GHC.Stack type AppendsCallStack = ?callStack::CallStack ==> Bar.hs <== {-# LANGUAGE FlexibleContexts #-} module Main where import AppendCallStack import MyError foo :: AppendsCallStack => a -> a foo x = myerror "Test" main = print (foo ()) ==> MyError.hs <== {-# LANGUAGE ImplicitParams #-} module MyError where import GHC.Stack myerror :: (?callStack :: CallStack) => String -> a myerror msg = error (msg ++ ": " ++ showCallStack ?callStack) Note that I need FlexibleContexts on the usage site to be able to use this, otherwise I get Non type-variable argument in the constraint: ?callStack::GHC.Stack.CallStack See it in action: $ ghc --make Bar && ./Bar Bar: Test: ?callStack, called at ./MyError.hs:7:51 in main:MyError myerror, called at Bar.hs:8:9 in main:Main foo, called at Bar.hs:10:15 in main:Main With GHC-HEAD, it compiles no longer(!): [1 of 2] Compiling AppendCallStack ( AppendCallStack.hs, AppendCallStack.o ) AppendCallStack.hs:6:1: error: • Illegal implicit parameter ‘?callStack::CallStack’ • In the type synonym declaration for ‘AppendsCallStack’ So Richard, does it do what you want with GHC-7.10? And given that GHC HEAD rejects it, was 7.10 wrong or is there a bug in HEAD? 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