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In F#'s printf there is the format specifier %A which enables to pass in any F# type, and it would be evaluated and printed.
As an example:
type Result<'a> =
| Failure
| Success of 'a
printf "%A" (Success "hello") // prints out 'Success "hello"'
Where, clearly, Result<'a> isn't a built-in type.
I can declare a similar type in OCaml, but there is no equivalent specifier for Printf.printf - instead, I would have to implement my own string_of_result function, and use the %s specifier in the format string. Moreover, since this is a polymorphic type, I would have to create a not-straightforward function that can handle any type instance of 'a.
My question is - why does OCaml lack this handy specifier? Is it because there is no incentive to implement it? Is it because there's some lacking under-the-hood mojo, which is just there in F#?
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Example 1: C Output The printf() is a library function to send formatted output to the screen. The function prints the string inside quotations. To use printf() in our program, we need to include stdio. h header file using the #include <stdio.
The %a formatting specifier is new in C99. It prints the floating-point number in hexadecimal form. This is not something you would use to present numbers to users, but it's very handy for under-the-hood/technical use cases. As an example, this code: printf("pi=%a\n", 3.14); prints: pi=0x1.91eb86p+1.
F-strings provide a way to embed expressions inside string literals, using a minimal syntax. It should be noted that an f-string is really an expression evaluated at run time, not a constant value. In Python source code, an f-string is a literal string, prefixed with 'f', which contains expressions inside braces.
In short, it is a way to format your string that is more readable and fast. The f or F in front of strings tell Python to look at the values inside {} and substitute them with the variables values if exists.
I'd say "lacking under-the-hood mojo" is probably the reason.
In F#, %A specifier defers printing to a reflection-based printer - it uses runtime type information to traverse and print the value. The reflection API used in that process is very much a .NET-specific thing. Also while handy, it's also a comparatively expensive mechanism - it shouldn't be used as a blanket specifier if you can use a more concrete one.
From what I know, OCaml doesn't have corresponding reflection capabilities that could be used here. Perhaps there's another mechanism that would let you implement a generic print - but I'm not familiar enough with OCaml internals to tell.
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