[Haskell-cafe] Improving HList programming/debugging (longish)

Wolfgang Jeltsch g9ks157k at acme.softbase.org
Wed Jan 12 10:12:21 CET 2011

Am Dienstag, den 11.01.2011, 20:05 -0500 schrieb jeff p:
> This message shows how to slightly reformulate HLists (and other
> type-level things) to get better type-checking and more informative
> error messages. The technique is interesting in that it uses GADTs and
> functional dependencies and seems to not be implementable with
> associated type synonyms. It also makes higher-order HList
> programming/debugging much more tractable.


a few years ago, I also experienced with using GADTs for representing
heterogenous lists. However, I dropped this idea, since GADTs force you
to provide type signatures in certain situations. For example, you
cannot leave out the type signatures in the following definitions:

> hHead :: HList (x :* xs) -> x
> hHead (x :* _) = x
> hTail :: HList (x :* xs) -> HList xs
> hTail (_ :* xs) = xs

This was at odds with a feature I wanted: It should be possible to match
a polymorphic value representing a family of heterogenous lists against
a pattern such that the type of the pattern specifies the concrete list
to use.

I finally switched back to the traditional approach used in today’s
HList. You can read about the pattern-matching thing in Subsection 5.4
of my paper “Generic Record Combinators with Static Type Checking” [1].

Best wishes,

[1] <http://www.informatik.tu-cottbus.de/~jeltsch/research/ppdp-2010-paper.pdf>

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