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Pitometsu/module_key.ml

Created August 21, 2019 14:29
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first-class module dependent types
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module_key.ml
let f (type k) : x:((module Caml.Map.S with type key = k) as 'a) -> y:'a -> z:k -> unit = fun ~x:(_) ~y:(_) ~z:(_) -> ();;
val f :
x:(module Core.Caml.Map.S with type key = 'a) ->
y:(module Core.Caml.Map.S with type key = 'a) -> z:'a -> unit = <fun>
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let get (type a) ~from:((module F) : (module Caml.Map.S)) : unit = let to_ : a F.t = failwith "undefined" in ignore to_;;
val get : from:(module Core.Caml.Map.S) -> unit = <fun>

But:

let get (type a) ~from:((module F) : (module Caml.Map.S)) ~to_:(m : a F.t) : unit = ();;
Error: This pattern matches values of type a F.t
       but a pattern was expected which matches values of type 'a
       The type constructor F.t would escape its scope

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