The HList code does not need overlapping-instances, however it does use
undecidable
instances. This is not however bad like overlapping instances is.
Overlapping instances
can break module independance (as in defining a new instance can change
the meaning
of an existing class in modules that are already compiled). Undecidable
instances merely
means the compiler is not capable of proving that the constraints
terminate. In the
case of an HList they "obviously" do (where the constraint recursion is
structurally over
the length of a list termination is obvious). This is more a weakness in
the compiler rather
than some problem with the HList code.
Keean.
Wolfgang Jeltsch wrote:
Am Dienstag, 22. November 2005 07:33 schrieb David Menendez:
Keean Schupke writes:
Haskell already has static records (in H98)
Dynamic records are addressed by the HList library, which uses
extensions already present in GHC and Hugs (namely Multi-parameter
type-classes and function-dependancies).
Is this the case? Every implementation of HList that I've seen also uses
overlapping and undecidable instances.
The paper about HList I have seen does explicitely say that the authors were
finally able to avoid using overlapping instances. I don't know about
undecidable instances but I thought (and hope very much) that they don't need
them too.
Best wishes,
Wolfgang
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