The following code fails to compile:<div><br></div><div>----- <a href="http://a.ml">a.ml</a> ----</div><div><div>module Foo = struct</div><div>  module St = struct type t = string let compare = compare end</div><div><br></div>

<div>  module Map (* : Map.S with type key = St.t *)</div><div>    = Map.Make(St)</div><div>  </div><div>  (* module Map = Map.Make(struct type t = string let compare = compare end) *)</div><div>end</div><div><br></div><div>

module Bar = struct</div><div>  type t = int Foo.Map.t</div><div>end</div><div><br></div><div>module Pack = struct</div><div>  module Foo = Foo</div><div>  module Bar = Bar</div><div>end</div><div><br></div><div>open Pack</div>

<div>let x : Bar.t = Foo.Map.empty</div></div><div>----</div><div><div><br></div><div>$ ocamlc -c <a href="http://foo.ml">foo.ml</a></div><div>File &quot;<a href="http://foo.ml">foo.ml</a>&quot;, line 20, characters 16-29: (* refers to Foo.Map.empty on last line *)</div>

<div>Error: This expression has type</div><div>         &#39;a Pack.Foo.Map.t = &#39;a Map.Make(<a href="http://Pack.Foo.St">Pack.Foo.St</a>).t</div><div>       but an expression was expected of type</div><div>         Pack.Bar.t = int Map.Make(Foo.St).t</div>

</div><div><br></div><div><br></div><div>So Pack.Foo and Foo are not compatible. By trial and error, I found two ways to resolve the problem:</div><div><br></div><div>1) Provide an explicit signature for Foo.Map (uncomment the provided signature), or</div>

<div>2) Inline the structure passed to Map.Make (comment out first definition of Foo.Map and uncomment the second one)</div><div><br></div><div>But I don&#39;t understand why. Can anyone explain the main points. Thank you.</div>

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