RFC: Add HasCallStack constraint to partial Data.List functions.

[Simon Peyton Jones]

|  HasCallStack is an operational annotation that has no place in declarative
|  type signatures

I'm not sure I really agree with that.  There is a rich literature on effect systems, which decorate types with information about what effects the function has: exceptions, divergence, IO, and the like.   So type like
   head :: Partial => [a] -> a
where 'Partial =>' expresses the fact that calling this function might lead to a call of 'error' doesn't seem inherently something that doesn't belong in a type system.

Indeed if `error :: Partial => String -> a`, then we can guarantee that any function that calls error will need a Partial constraint... it propagates upward as it should.

TL;DR none of this seems (to me) inappropriate for a declarative type system.

Simon