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13071Re: "ocaml_beginners"::[] I need help deciphering Martin's hash table code

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  • Dan Bensen
    Jan 1 7:28 PM
      > The type definition on the left side of the "=" sign is the same as in the
      >OCaml Hashtbl reference.

      That just means they both take two type arguments (and have the same name).
      Hashtbl2.t is a new type.

      > The right side of the "=" sign indicates that of any two types that are a hash,
      >the second is bound as a list reference.

      The right side is a description of the new type.
      'a is the type of the keys in a value of type Hashtbl2.t, and ('b list ref) is
      the type of the values.

      > The problem I am having is understanding the segment "let r =" in the add
      >function.

      First r is bound to a list ref, and then data is pushed onto it.
      This is the overall structure of the function:

      let add tbl key data = let r = ... in r := data :: !r

      This code should be equivalent (untested):

      (* add a new empty entry if there isn't one already,
      and return the value *)
      let find_or_make tbl key =
      try Hashtbl.find tbl key
      with Not_found ->
      let r = ref [] in
      Hashtbl.add tbl key r;
      r

      (* push data onto the (possibly new) entry for key *)
      let add tbl key data =
      let r = find_or_make tbl key in
      r := data :: !r

      > When adding a key to the hash table, first we check to see if it already exists
      >in the table

      When pushing a new element onto the *value* (which is a list ref), we first
      check to see if there's an entry for the *key*.

      > if it is not found, we treat it as a list reference

      If it's not found, we make a new one. We always treat the values as list refs,
      because that's what they are by the definition of Hashtbl2.t.

      > Why are we assigning the "try...with" block to the variable 'r'?

      r is bound to the value *returned* by the try expression.
      If tbl already has an entry for key,
      the try expression returns the value of (Hashtbl.find tbl key).
      If there's not an entry, the try expression returns the value of
      (let r = ref [] in Hashtbl.add tbl key r; r)

      > Do we need to emit some kind of a message that the key already exists in the
      >hash table

      No, the try form just returns the value.

      > or are we simply going to overwrite the value?

      The code overwrites the value in tbl after it creates a list with data consed
      onto the original list (or the new empty one).

      > What about the second 'let r = ref [] in", is it shadowing the variable 'r'?

      Yes, temporarily. That code is executed only if there's no pre-existing entry.

      [Non-text portions of this message have been removed]
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