Question

Why does this behavior occur?

# Printf.sprintf ("Foo %d %s") 2 "bar";;
- : string = "Foo 2 bar"

# Printf.sprintf ("Foo %d" ^ " %s") 2 "bar";;
  Printf.sprintf ("Foo %d" ^ " %s") 2 "bar";;
Error: This expression has type string but an expression was expected of type
         ('a -> 'b -> 'c, unit, string) format =
           ('a -> 'b -> 'c, unit, string, string, string, string) format6

I would expect that the string concatenation would be evaluated first, so everything will proceed as normal. Does this have to do with the type system trickery that Printf employs?

Answer 1

Yes, it has to do with type system trickery. If you want to create a format string you need to use the (^^) operator:

# Printf.sprintf ("Foo %d" ^^ " %s") 2 "bar";;
- : string = "Foo 2 bar"

I’m not deeply schooled in this trickery, but I believe that the compiler is willing to promote a string constant to a printf format if the typing context calls for it. However, the result of ("Foo %d" ^ " %s") is not a string constant, so it doesn’t get promoted. The (^^) operator creates a typing context where both operands can be promoted if they are string constants.

You can see why it would have to be a string constant: otherwise the associated types (of the values to be printed) can’t be determined.