Hash collisions in OCaml polymorphic variants
Polymorphic variants in OCaml compile down to integers (if they
don’t have arguments). As opposed to IDs chosen sequentially for
non-polymorphic variants, these integers are chosen by hashing the
value’s name. For example, value `Foo is given the
integer value 3505894.
As with any hashing algorithm, the algorithm used here is subject to
collisions, for example according to this thread on the Caml mailing
list values `Eric_, `azdwbie and `c7diagq all hash to
integer value -332323982.
Thankfully this will not cause issues in practice as the OCaml compiler is smart enough to fail whenever collisions occur within a polymorphic variant type. Trying this with the OCaml REPL fails as follows.
# type collision = [`Eric_Cooper | `azdwbie];;
Error: Variant tags `azdwbie and `Eric_Cooper have the same hash value.
Change one of them.