51 lines
1 KiB
OCaml
51 lines
1 KiB
OCaml
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(* Sec 01, Feb 3 *)
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(*
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fold (+) 0 [1;2;3;4]
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We can see this as
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1 + (2 + (3 + (4 + 0)))
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*)
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let rec fold_v1 f e l =
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match l with
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| [] -> e
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| x::xs -> f x (fold_v1 f e xs)
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let f : (int -> string -> string) =
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fun x -> fun s -> string_of_int x ^ ", " ^ s
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(* fold (+) 0 [1;2;3;4]
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as
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((((0 + 1) + 2) + 3) + 4)
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*)
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let rec fold_v2 f e lst =
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match lst with
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| [] -> e
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| hd::tail -> fold_v2 f (f e hd) tail
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let g : (string -> int -> string) =
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fun s -> fun x -> s ^ ", " ^ string_of_int x
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(* Below are the versions of these functions from the slides. They
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are the same excpet for their names. Below we chose 'l' and 'r' to
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indicate if we are folding lists up from the left of from the
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right. *)
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let rec foldr (f:'a -> 'b -> 'b) (l:'a list) (v:'b) : 'b =
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match l with
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| [] -> v
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| x::xs -> f x (foldr f xs v)
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(* Notice how 'f' replaces '::' and 'v' replaces '[]' *)
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let rec foldl f v l =
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match l with
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| [] -> v
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| x::xs -> foldl f (f v x) xs
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let length (lst: 'a list) : int = foldl (fun n _ -> n + 1 ) 0 lst
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