I am reading a book on OCaml, I am new to OCaml so I am doing the exercises and examples-though not everything compiles. In this case, my project tonight is an example of a hash table which compiles and is very interesting but I can't get it working.
let random_numbers =
[|0x316666; 0x2d331c; 0x4de673; 0x63a91e; 0x87bee8|]
let random_length = Array.length random_numbers
type hash_info = { mutable hash_index : int; mutable hash_value : int }
let hash_char info c =
let i = Char.code c in
let index = (info.hash_index + i + 1) mod random_length in
info.hash_value <- (info.hash_value * 3) lxor random_numbers.(index);
info.hash_index <- index
let hash s =
let info = { hash_index = 0; hash_value = 0 } in
for i = 0 to String.length s - 1 do
hash_char info s.[i]
done;
info.hash_value
type 'a hash_entry = { key : string; value : 'a }
type 'a hash_table = 'a hash_entry list array
let create () =
Array.make 101 []
let add table key value =
let index = (hash key) mod (Array.length table) in
table.(index) <- { key = key; value = value } :: table.(index)
let rec find_entry key = function
{ key = key' ; value = value } :: _ when key' = key -> value
| _ :: entries -> find_entry key entries
| [] -> raise Not_found
let find table key =
let index = (hash key) mod (Array.length table) in
find_entry key table.(index);;
The author unfortunately will say things like 'a string -> unit will do this or that when such is not actually the case. Here for example is what I am dealing with:
utop # add = { key = "9999999" ; value = "one of these days" };;
Line 1, characters 6-55:
Error: This expression has type string hash_entry
but an expression was expected of type
'a hash_entry list array -> string -> 'a -> unit
I am getting frustrated with these extensive statements. What does this mean?
'a hash_entry list array -> string -> 'a -> unit
I am not asking for a definition of the parts of it, how do I use this statements to call the function? I understand that understanding the parts of this is important of course, how do these things work? A couple of days a go, for another example I was stuck on expecting type unit, what on earth is "type unit" supposed to be? Specifically, how can I generate this (knowing that it expects a key/value pair):
'a hash_entry list array -> string -> 'a -> unit
Here is the expression you are having trouble with:
add = { key = "9999999" ; value = "one of these days" }
This is an expression, but it's not a call to add. add is a function of 3 parameters, so a call would look like this:
add mytable mykey myvalue
Instead this expression is a comparison, i.e., it tests whether two things are equal. That's because the only operator in the expression is =, the equality comparison operator.
Since add is a fuction, it isn't even the right type to compare against a record, which is what appears on the right side of the comparison. So that's why you're getting an error from the interpreter.
If you want to test a call to add, it would look something more like this:
# let table = create ();;
val table : '_weak1 list array = ...
# add table "mykey" "myvalue";;
- : unit = ()
# find table "mykey";;
- : string = "myvalue"