CODEWITHSUNDEEP
ocamltyping
ocamlnewb
ocamlnewb

Reputation: 1

Why does Ocaml throw this "expression was expected of type int" error only when a function is applied on negative integers

Let me explain what I'm trying to do first. I'm learning about Ocaml types, and I define a new type, say int2 that is the same as int.

# type int2 = int;;
type int2 = int

So far so good.

Now I want to define a function called add_five that take an argument of type int and returns a value of type int2

# let add_five (x : int) = (x + 5 : int2);;
val add_five : int -> int2 = <fun>

Great! Now I want to apply this to one positive number, and one negative number to confirm that it works correctly.

# add_five 5;;
- : int2 = 10

That worked correctly!

# add_five -7;;
Error: This expression has type int -> int2
       but an expression was expected of type int

What? I don't understand why this is happening. I explicitly told Ocaml I wanted a return function of type int2, so why is it claiming it needed to be type int?

Upvotes: 0

Views: 256

Answers (1)

ivg
ivg

Reputation: 35210

add_five -7 is parsed as (add_five) - (7), i.e., the - operator is infix. So add_five is expected to be something from which you can subtract 7, i.e., a value of type int. The solution is either to parenthesize it (-7) or use the infix form of the negation ~-7

Upvotes: 4

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