Is [] a type or not?

Learning
21

I understand why let () = 1 + 2 leads to an error message when () is redefined. My question is why utop would be rewriting 1 + 2 as let () = 1 + 2, since it seems that the latter should lead to an error message even when () isn’t redefined.

22

The full rewriting rule for utop is that is first evaluated as

let _ () = let module _ = struct <expr> end in ()

in order to check the expression without evaluating it.

23

Slightly off-topic, but another way I find redefining [] and (::) useful locally is when combining them with GADTs and existential types to implement kinds of DSLs.

A recent example was for heterogenous pretty printing of parameters:

    let module M = struct
      type t =
        | [] 
        | (::) : ((formatter -> 'a -> unit) * 'a) * t -> t

      let rec pp_ls fmt = function
        | [] -> ()
        | (pp, v) :: [] -> pp fmt v
        | (pp,v) :: tl -> pp fmt v; pp_print_string fmt ","; pp_print_space fmt (); pp_ls fmt tl
    end in

and then instantiating them:

(pp_rule "Read" M.[pp_print_string, var; pp_print_int, offset])

(where var is of type string, and offset of type int).

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