Ben Rudiak-Gould wrote:
Matthias Fischmann wrote:
now i want to create a list of a type similar to
[r1, r2, r3] :: (Resource a) => [a]
but with r1 being pizza, r2 being crude oil, and so on.
The type you actually want here is [exists a. (Resource a) && a], but no Haskell implementation supports that.
Is there a reason for using && instead of [exists a. Resource a=>a] ? Also, am I right in thinking that the reason why no implementation supports this is as follows: a) A value 'v' of type 'exists a. Resource a=>a 'would have to be internally represented as something like (dictResource_t, value_t) and b) Given such a value, there is no syntactic way to distinguish the pair from the value_t stored inside it, unlike the use of 'forall' where the syntax for the constructor conveniently "represents" any dictionaries that have been glued onto the value (ie pattern matching against R x gives us back the dictionaries "R" and the plain value x)? Hope I'm not making this more confusing but I'm still trying to get my head around all these invisible happenings regarding dictionaries so I can visualise what's happening in terms of bytes and pointers in the runtime.... Thanks, Brian.