[bitc-dev] Haskell question: exceptions

[Jonathan S. Shapiro]

On Fri, 2005-08-12 at 11:29 -0700, Iavor Diatchki wrote:

> I think the original ML value restriction said that only expressions
> that syntactically look like values can be polymorphic.  Then it is
> surprising that the following works:
> 
> # let r = if true then [] else raise Xn;;
> val r : 'a list = []
> 
> because the expression is not a value, and yet it became polymorphic....

Yes, it is coming back to me now. SML requires that a binding can be
treated as polymorphic only if it is initialized by a "syntactic
constant." I looked at one point, and I was not able to find a rigorous
definition of syntactic constant, but the intuition is that the value of
the initializing expression must be computable at compile time.

So what is happening in the case above is that the entire expression is
being reduced at compile time to "[]", which is polymorphic.

So now the question is: what is the difference between the following two
cases that causes the value restriction to be invoked in one but not the
other:

# let r = if true then [] else raise Xn;;
val r : 'a list = []

# let q = if true then id else raise Xn;;
val q : '_a -> '_a = <fun>

Clearly there is something different in the handling of type
constructors ("[]") vs. functions.

shap