Friday, 4 May 2012

Map and fold functions that are generic over the kind of collection

There are two main solutions in OCaml:
  1. Jacques Garrigue already implemented a syntactically-light but inefficient approach for many data structures several years ago. You just wrap the collections in objects that provide a map method. Then you can do collection#map to use the map function for any kind of collection. This is general because it allows different kinds of data structures to be substituted at run time. However, this is not very useful in practice so the approach was never widely adopted.
  2. A syntactically-heavier but efficient, robust and static solution is to use functors to parameterize your code over the data structure you are using. This makes it trivial to reuse your code with different data structures. See Markus Mottl's OCaml translations of Okasaki's book "Purely Functional Data Structures" for some great examples.
If you aren't looking for that kind of power and just want brevity then, of course, you can just create a module alias with a shorter name (e.g. module S = String).

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