Simon Peyton Jones
simonpj at microsoft.com
Wed Jan 20 10:25:47 UTC 2016
| 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?
Hmm. Actually I think that's quite a good idea.
The call-stack idea started here https://ghc.haskell.org/trac/ghc/wiki/ExplicitCallStack/ImplicitLocations. We want to be able to get hold of the current call stack. It obviously doesn't make sense to say
currentCallStack :: CallStack
But as the call stack is an implicit parameter (operationally), it obviously DOES make sense to say
?currentStack :: (?currentStack :: CallStack) => CallStack
if ?currentStack is an implicit parameter. And it went from there.
There are disadvantages:
* Need to use -XImplicitParams (Richard's point)
* Need to align two names:
foo :: (?loc :: CallStack) => Int -> Int
foo x = if x<0 then error "urk" else -x
won't work, because 'error' needs (?callStack :: CallStack)
* The special cases in the type checker need a 2-level pattern
match: for the magic "IP" class, and then the magic "CallStack" type
* In principle you might have multiple call stacks kicking around
at the same time
boo :: (?a::CallStack, ?b::CallStack) => Int -> Int
Now I'm not really sure what is supposed to happen about solving
these constraints. Perhaps it could be a feature, but it's not
one anyone has asked for, and even having to think about it makes
my head hurt.
Your alternative suggestion is to have a magic nullary class, the ICallStack class ("I" for implicit) so that
class ICallStack where
callStack :: CallStack
At least that's is the implementation, but all the user can see is the overloaded function
callStack :: ICallStack => CallStack
The solving rules, the CallStack type, and functions for printing it, would be precisely as now.
I like this. What about others?
(We'd have to think about what to do for 8.0 but first lets see what we want.)
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