[mercury-users] Maths and contexts.

[Paul Bone]

On Sat, Apr 21, 2012 at 01:33:28AM +1000, Julien Fischer wrote:
> 
> On Sat, 21 Apr 2012, Julien Fischer wrote:
> 
> >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.
> 
> You can support construction of contexts at runtime and get compile-time
> checking of contexts mismatches (at the cost of adding an extra word to
> the representation of a number) using existential types.  The modified
> decimal module below demonstrates this -- I left most of the fields out
> of the context to save space.  The following test program attempts
> to add two decimals that have different contexts associated with them:

Now that there is a value of C in decimal(C),  the typeclass can be defined in
terms of C rather than decimal(C).

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