Implicit Parameters

Jeffrey R. Lewis
Mon, 4 Feb 2002 09:14:54 -0800

On Monday 04 February 2002 02:25 am, John Hughes wrote:
> Not so fast! Proposing a "solution" means this is regarded as a "problem"!
> But what is to say that the first behaviour is "right" in any general
> sense?
> The important thing is that the language semantics is clear, and this is a
> semantic question.  The static semantics of Haskell *is* clear: recursive
> calls are monomorphic unless a type signature is given; this is the basis
> for understanding the behaviour above. 

I think part of the problem is that we've confused implicit parameterisation 
with polymorphism.  Haskell has a two-level type system with monomorphic 
types at the bottom level, and polymorphic and qualified types at the second 
level.  It turned out to be very straightforward to add implicit parameters 
to Haskell by treating them as a special kind of qualified type, and thus 
they also play according to the rules of polymorphic types - i.e. you 
`capture' implicit parameters exactly when you generalize a polymorphic 

However, Koen's example suggests that maybe implicit parameters shouldn't 
necessarily play according to the rules of polymorphic types.  Perhaps 
implciit parameters should be a special kind of monomorphic type instead.  If 
this were the choice, then it's clear that they should be captured by 
recursive definitions.

> When implicit parameters are used,
> it's very important to be aware whether a binding is monomorphic or not
> (can't resist plugging := again!). Will your "solution" make understanding
> when a binding is monomorphic simpler? If not, it could be worse than the
> "problem" -- and the fact that it makes this one example behave as you want
> is no justification.

I agree that we should tread carefully ;-)