[mercury-users] Maths and contexts.

[Paul Bone]

On Sat, Apr 21, 2012 at 12:43:38AM +1000, Julien Fischer wrote:
> Specifically, how does a given context type get associated with the result
> value here?  Since C is universally quantified it must be passed in
> by the caller, but nothing else in this API that will provide a binding
> for it.
> 
> The signature for to_number actually needs to be something like:
> 
>   :- func to_number(C, string) = dec(C) <= context(C).
> 
> i.e. at some point you need to actually associate a particular context
> with a (set of) dec/1 value(s).

I was thinking that "it would be constrained by the type system" and didn't
worry about it anymore, forgetting that you'd either have to construct a value
of this type or use a type declration as Mark suggested.

> The other limitation of this approach is that you are limited to the
> set of context types that are defined in your program.  You cannot
> construct new contexts at runtime.
> 

Thanks, I didn't notice these problems.

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