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