Shadowing types in OCaml

One weird thing in OCaml is that you can shadow types, just like you can with variables.

Shadowing types is NOT the same as overriding a type.

Here is an example:

let x = 3
type int = OtherType
let y = OtherType
let z = 10
let w : int = 2

Here, the original int type technically still exists, but you just can't access it through the type int anymore. z can still be defined, because OCaml can still infer its type to be the original int type, but we can't define w like that using the int annotation, because the annotation now points to a different type.

I urge you to verify these statements to be true on utop or something similar.

Utop experiments

Utop can show some weird things when you shadow types. I will overshadow ints again as an example. Check out the following experiment in utop. The curious thing is that here, since the original int type got overshadowed twice, typing in an integer will say it's got type int/3, because it matches the type of the third-to-last int definition.

──────────┬─────────────────────────────────────────────────────────────┬───────────
          │ Welcome to utop version 2.16.0 (using OCaml version 5.3.0)! │
          └─────────────────────────────────────────────────────────────┘

Type #utop_help for help about using utop.

─( 00:11:41 )─< command 0 >──────────────────────────────────────────{ counter: 0 }─
utop # type int = A;;
type int = A
─( 00:11:41 )─< command 1 >──────────────────────────────────────────{ counter: 0 }─
utop # let a = A;;
val a : int = A
─( 00:11:56 )─< command 2 >──────────────────────────────────────────{ counter: 0 }─
utop # type int = B;;
type int = B
─( 00:11:59 )─< command 3 >──────────────────────────────────────────{ counter: 0 }─
utop # let b = B;;
val b : int = B
─( 00:12:03 )─< command 4 >──────────────────────────────────────────{ counter: 0 }─
utop # a;;
- : int/2 = A
─( 00:12:06 )─< command 5 >──────────────────────────────────────────{ counter: 0 }─
utop # 10;;
- : int/3 = 10
─( 00:12:07 )─< command 6 >──────────────────────────────────────────{ counter: 0 }─

Video demo