Günther Schmidt wrote:
Hi Stephen,
I'm glad I asked. This sure sounds more interesting than I had
anticipated. Is this an old hat for your off-the-shelf haskeller or
something only found in the more seasoned haskellers tool box?
I think it's pretty much the first time I encounter it.
It depends on what kind of seasoning you take :)
Another use case which is really common is to use open recursion for
things like memoization. That is, if we write an open-recursive
function, using `fix` turns it into a normal recursive function but we
can use a different fixed-point combinator in order to memoize the
results. Luke Palmer has packaged up a version of this on Hackage[1].
You can perform similar tricks on the type level. Wouter Swierstra
combats the expression problem by using open recursion to build ad-hoc
union types[2]. Whereas Tim Sheard uses a somewhat different version of
open recursion to create parameterized modules[3], with a specific use
case being to separate variables from structure in unification
algorithms[4].
[1] http://hackage.haskell.org/package/data-memocombinators
[2] http://www.cs.nott.ac.uk/~wss/Publications/DataTypesALaCarte.pdf
[3] http://web.cecs.pdx.edu/~sheard/papers/JfpPearl.ps
[4] http://web.cecs.pdx.edu/~sheard/papers/generic.ps
--
Live well,
~wren
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