What are the biggest reasons newcomers give up on OCaml?

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the OCaml language itself has been relatively easy to understand (so far!).

I’d like to throw down a gauntlet.

Here is four lines of working F# code that I can write off the top of my head that represent a regular expression that makes it easy to implement Antimirov derivatives:

type RegEx<'a when 'a: comparison> =
  | Lit of 'a
  | Alt of Set<RegEx<'a>>
  | Seq of RegEx<'a> list

That code means “I have a type RegEx that is polymorphic and parameterised over a type variable 'a with the constraint that 'a implements equality and comparison. A RegEx is either a literal 'a or one of a set of alternative RegExs or a sequence of RegExs”.

Compiled and ran first time. Succinct. Comprehensible. Nice.

Some notes:

  • Being polymorphic over the element type means the same code can be used to match sequences of any type, not just strings of chars or arrays of bytes.
  • Using sets to represent alternatives and lists to represent sequences means the regular expression that doesn’t match any input is bot = Alt Set.empty and the one matching with no input is eps = Seq []. This is simpler than having more cases: Bot | Eps | Alt of re * re | Seq of re * re.
  • The use of a sorted set of regular expressions eliminates duplicates and improves asymptotic performance which is more efficient than using explicit cases.
  • You can print any value of any type including thisRegEx using printfn "%A".

I’d love to see a simple port of this to OCaml. I just spent an hour trying and my use of mutually-recursive higher-order modules with PPX deriving show and ord resulted in pages of code that still doesn’t work.

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