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I guess I’ve been living under a rock for the last year, but I’ve only recently started using a couple of fluent APIs. A quick search of the internet and fluent APIs are popping up for everything from twitter to orm mapping to dependency injection. So far I buy into the advantages of fluent interfaces over [...] We all know the importance of protecting consumers of our libraries from our design decisions. In object oriented programming we refer to this protection mechanism as encapsulation or black box design. Functional programming allows us to achieve the same goal under a slightly different guise, abstract [...] When pitching functional programming to an imperative friend, the “map” function is the canonical example of the obvious advantages of the functional approach. And once our backwoods imperative friend has begrudgingly accepted “map” as a neat party trick, we can really amaze them by abstracting “map” [...] When a book is packed with concepts and information it’s very easy to gloss over something that in retrospect is very interesting. I often skim back over things I’ve already read hoping some of the new information I’ve accumulated will lead to deeper insights and interesting [...] If you’ve been programming in F# for any length of time then undoubtedly you’ve run into the term “call by value”. Another name for “call by value” is eager evaluation as opposed to lazy evaluation and the corresponding “call by name”. Let’s throw together a fun [...] We left off in a rather unsatisfying predicament, we can simulate OCaml functors with interfaces, but at the expense of passing in two type parameters and guarding all our functions with type constraints. Yuck!
The following page describes in detail the differences between the OCaml and F# languages. It’s a good read for anyone familiar with OCaml and an even better read for those who are not. For the first line of the summary declares
The map function is one of the most useful functions in all of functional programming. Pass a function, a list of arguments, apply the function to each argument, and return a new list of the results.
We used a good old fashioned subtyping and a dash of parametric polymorphism to write an eval function capable of returning different types. But surely we gave up too easily on polymorphic return types in F#? I assure you we gave them the ol’ college try.
So we know how to use a type parameter to statically constrain the length of a list. Let’s continue our exploration of type constraints and see if we can figure out how to constrain the return type of a data constructor.
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