Question

The following code does not compile:

let x = "hello" in
Printf.printf x

The error is:

Error: This expression has type string but an expression was expected of type
     ('a, out_channel, unit) format =
       ('a, out_channel, unit, unit, unit, unit) format6

1) Can someone give an explanation of the error message?

2) And why would a string cannot be passed to printf ?

Answer 1

The first argument to printf must be of type ('a, out_channel, unit) format not string. String literals can be automatically converted to an appropriate format type, but strings in general can’t.

The reason for that is that the exact type of a format string depends on the contents of the string. For example the type of the expression printf "%d-%d" should be int -> int -> () while the type of printf "%s" should be string -> (). Clearly such type checking is impossible when the format string is not known at compile time.

In your case you can just do printf "%s" x.