[Haskell-cafe] Constraints as a moveable feast?

Anthony Clayden anthony_clayden at clear.net.nz
Sun Sep 13 07:13:11 UTC 2020

On Thu, Sep 3, 2020 at 2:18 PM Anthony Clayden <anthony_clayden at clear.net.nz>

> > on *Tue Sep 1 21:18:40 UTC 2020, Richard Eisenberg wrote:*
> > I see the core idea as a fairly straightforward generalization of the >
> "Partial Type Constructors" work -- except that I wouldn't phrase any of >
> this as "floating out".

Heh heh as a wee sidenote to the idea of type synonyms/datatypes making
their contexts available, as you put it

| > So, the occurrence of Point a causes a Num a constraint to arise.
These Num a constraints then get put into the type.

The 1991 Haskell report that dropped type synonym contexts and
specified datatype contexts as we now know them, left something
under-specified. SPJ/GHC interpreted it one way, Mark P. Jones/Hugs
interpreted differently. It didn't come to light until the H98 report:


"Phil (Wadler), Mark (P. Jones), Jeff (Lewis)" (and Erik Meijer later
in the thread) preferred Hugs's approach, so that went into the
standard and into GHC. Phil said "`redundant pollution' is exactly the
effect I want to achieve!".

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