CODEWITHSUNDEEP
ocaml
K.WANG
K.WANG

Reputation: 71

How should I check whether a number is in a list in OCaml?

Why is my code wrong?

# let ls = [1;2];;

val ls : int list = [1; 2]

# let inList a l = List.exists a l;;

val inList : ('a -> bool) -> 'a list -> bool =

# inList 1 ls;;

Error: This expression has type int but an expression was expected of type 'a -> bool

Upvotes: 1

Views: 12676

Answers (2)

Lhooq
Lhooq

Reputation: 4441

Well, as you can see :

# let inList a l = List.exists a l;;

val inList : ('a -> bool) -> 'a list -> bool

So a is of type 'a -> bool which means that a is a predicate on each element of the list.

What you wanted to write was

let inList a l = List.mem a l

val inList : 'a -> 'a list -> bool

TL;DR RTFM ;-) http://caml.inria.fr/pub/docs/manual-ocaml/libref/List.html

Upvotes: 1

Jeffrey Scofield
Jeffrey Scofield

Reputation: 66818

The first parameter of List.exists is a function that returns true if the element is one you're looking for and false if not. You're supplying the int 1, which isn't a function.

You need a function looking_for like this:

let inList a l =
   let looking_for x = ... in
   List.exists looking_for l

The function looking_for should return true if x is what you're looking for (i.e., if it's equal to a) and false otherwise.

Upvotes: 2

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