title: type +'a t in Ocaml Map Library?
body: <p>I'm working with Ocaml's built-in Map library for a problem set, and I'm having trouble accessing the datatype of a map itself. This is supposed to be the third implementation of a dictionary (the first two being a list and an unbalanced binary search tree), and part of the functor I have to implement is "type dict", which is the datatype of the actual dictionary. For list, type dict was (D.key * D.value) list; for the tree, type dict was Empty | Branch((D.key * D.value), dict, dict). The Ocaml documentation says:</p>
  
  <pre><code>type +'a t 
  The type of maps from type key to type 'a.
  </code></pre>
  
  <p>This seems like what I need, but I can't seem to use it correctly. M is my Map.Make module, by the way. I've tried</p>
  
  <pre><code>type dict = M.t
  type dict = M.+D.value t
  type dict = M.+
  </code></pre>
  
  <p>But I keep getting error messages. Can anyone help? Thanks so much!</p>