Incoherence

[John Launchbury]

John,

As I recall, this was something we addressed in the original paper, 
indicating the extreme danger of the monomorphism restriction in the 
context of implicit paramters. They simply don't mix.

Down with the MR. Let's settle on a viable alternative as soon as possible.

On the more general topic of reasoning, I am more and more coming to the 
conclusion that we ought to see Haskell as an explicitly typed language, 
but one in which the compilers provide a standard set of heuristics to 
deduce which explicit types are present. Thus the (dynamic) semantics 
for Haskell are always to be given in the context of full explicit 
typing. This provides a separation of concerns between the intricacies 
of type inference (which can almost always be overcome if the heuristics 
are misbehaving with respect to programmer intent), and the relatively 
simple underlying semantics.

John



John Hughes wrote:

> I noticed today that the presence or absence of a type signature can
> change the RESULT of an expression in Hugs and GHC nowadays. Here's an
> example:
> 
>     a = (let x = ?x in
> 	 x with ?x = 1)
> 	with ?x = 2
>     -- a == 2
> 
>     b = (let x :: (?x :: Integer) => Integer
> 	     x = ?x in
> 	 x with ?x = 1)
> 	with ?x = 2
>     -- b == 1
> 
> It's the infamous monomorphism restriction at work, again, of course. Now,
> what are the proof rules for reasoning about implicit parameters again (:-)?
> 
> John Hughes
> 
> 
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