Re: add a new equation to Data.Type.Bool.If

If you want a laundry list, there's an exhaustive set of normal forms in 'normalized' here: https://github.com/ekmett/coda/blob/b278ceab217236f24a26e22a459d4455addd40db/lib/bdd/Data/BDD.hs#L266\ which is used to shrink the size of my 'if-then-else' lookup tables for BDDs. You don't need the normal forms per se, (and getting them requires some notion of ordering we can't offer), but you may find those and the base cases at https://github.com/ekmett/coda/blob/b278ceab217236f24a26e22a459d4455addd40db... to be useful at reducing the amount of stuff you need to compute. -Edward On Fri, Dec 29, 2017 at 10:34 AM, David Feuer wrote:

Heh. I already wrote the Phab differential weeks ago. But then I noticed there's room for more equations, and wasn't sure where to stop.

If x x False = x If x True False = x If x True x = x

On Dec 29, 2017 10:27 AM, "Richard Eisenberg" wrote:

...

Currently, we have (in Data.Type.Bool):

...

-- | Type-level "If". @If True a b@ ==> @a@; @If False a b@ ==> @b@ type family If cond tru fls where If 'True tru fls = tru If 'False tru fls = fls

I propose adding a new equation, thus:

...

-- | Type-level "If". @If True a b@ ==> @a@; @If False a b@ ==> @b@ type family If cond tru fls where If b same same = same If 'True tru fls = tru If 'False tru fls = fls

This new equation would allow If to reduce when we don't know the condition but we do know that both branches are the same. All three equations are *compatible* (a technical term defined in the closed type families paper), meaning that GHC ignores the ordering between them and will just choose whichever equation matches.

Any objections?

Thanks, Richard _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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