CODEWITHSUNDEEP
pattern-matchingocamltuples
Jack
Jack

Reputation: 133567

Matching of tuples

From what I understood I can use pattern-matching in a match ... with expression with tuples of values, so something like

match b with
  ("<",  val) -> if v < val then true else false
| ("<=", val) -> if v <= val then true else false

should be correct but it gives me a syntax error as if the parenthesis couldn't be used:

File "ocaml.ml", line 41, characters 14-17: Error: Syntax error: ')' expected

File "ocaml.ml", line 41, characters 8-9: Error: This '(' might be unmatched

referring on first match clause..

Apart from that, can I avoid matching strings and applying comparisons using a sort of eval of the string? Or using directly the comparison operator as the first element of the tuple?

Upvotes: 2

Views: 2684

Answers (2)

Chris
Chris

Reputation: 36496

It's not the core of your problem, but a more flexible (and potentially performant as the lookup map grows) approach to this problem may be to use a map of strings to functions, and then use the lookup functions in the map to handle the dispatch.

module StrMap = Map.Make (String)

# let eval_binary_op op a b =
    StrMap.(
      let m =
        empty
        |> add "<" (<)
        |> add "<=" (<=)
      in
      match find_opt op m with
      | None -> failwith "op not found"
      | Some f -> f a b
    );; 
val eval_binary_op : string -> 'a -> 'a -> bool = <fun>
# eval_binary_op "<" 4 5;;
- : bool = true
# eval_binary_op "<" 7 5;;
- : bool = false
# eval_binary_op ">" 7 5;;
Exception: Failure "op not found".

Upvotes: 0

sepp2k
sepp2k

Reputation: 370112

val is a reserved keyword in OCaml, so you can't use it as a variable name. If you use something else instead of val, it will work.

As a side note: if condition then true else false is equivalent to condition.

Upvotes: 9

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