testingPrintfFormat.fsx

// getting to grips with PrintfFormat

//let sscanf (pf:PrintfFormat<_,_,_,_,'t>) s : 't =

let explorePF (pf:PrintfFormat<'printer,'state,'residue,'result,'tuple>) s : string =
    pf.Value

explorePF "(%s %% % %i)" "(somestring)"
explorePF "(%s %% %z %i)" "(somestring)"    // error FS0741: Unable to parse format string 'Bad format specifier: 'z''

let explorePFGeneric (pf:PrintfFormat<'printer,'state,'residue,'result,'tuple>) s : 't =
    pf.Value // has a little moan about 't being constrained to be less generic since .Value returns a string

explorePFGeneric "(%s %% % %i)" "(somestring)"

(* from page 75 of the F# spec 3.1
More concretely, a constant string is interpreted as a printf-style format string if it is expected to have the type Microsoft.FSharp.Core.PrintfFormat<'Printer,'State,'Residue,'Result,'Tuple>. The string is statically analyzed to resolve the generic parameters of the PrintfFormat type, of which 'Printer and 'Tuple are the most interesting:
•	'Printer is the function type that is generated by applying a printf-like function to the format string.
•	'Tuple is the type of the tuple of values that are generated by treating the string as a generator (for example, when the format string is used with a function similar to scanf in other languages).
A format placeholder has the following shape:  
%[flags][width][.precision][type]

*)

// set 'printer?
// let's say we expect 'printer to deliver a string - entirely reasonable
let ``explorePF with string to string`` (pf:PrintfFormat<(string -> string),'state,'residue,'result,'tuple>) s : string =
    pf.Value

``explorePF with string to string`` "((%s %s))" "((1 string))"
(* Type mismatch. Expecting a
    string -> string    
but given a
    string -> string -> 'a    
The type 'string' does not match the type 'string -> 'a'
*)

``explorePF with string to string`` "((%s))" "((1 string))" // works nice

let ``explorePF with int and string to string`` (pf:PrintfFormat<(int -> string -> string),'state,'residue,'result,'tuple>) s : string =
    pf.Value

``explorePF with int and string to string`` "--%i %s some other stuff" "123 and"
``explorePF with int and string to string`` "--%i %s some other stuff" "123 123"
``explorePF with int and string to string`` "--%i %s some other stuff" ""
``explorePF with int and string to string`` "--%i %s some other stuff" //gives an error
(*
Value restriction. The value 'it' has been inferred to have generic type
    val it : ('_a -> string)    
Either make the arguments to 'it' explicit or, if you do not intend for it to be generic, add a type annotation.
*)

// so with the 'Printer we can enforce some type checking on the format parameters but so far we're not getting access to the actual string

(*
Also, note example from spec on page 76

"For example, the format string "%s %d %s" is given the 
type PrintfFormat<(string -> int -> string -> 'd), 'b, 'c, 'd,(string * int * string)> for fresh variable types 'b, 'c, 'd. Applying 
printf to it yields a function of type string -> int -> string -> unit."

*)
let ``explorePF with int and string to string and tuple`` (pf:PrintfFormat<(int -> string -> string),'state,'residue,'result,(int * string)>) s : string =
    pf.Value

``explorePF with int and string to string and tuple`` "--%i %s some other stuff" "123 and"
``explorePF with int and string to string and tuple`` "--%i %s some other stuff" "123 and 123"
``explorePF with int and string to string and tuple`` "%i %s %s some other stuff" "123 and 123"

// let's simplify
fun _ -> "%i %s":PrintfFormat<(int -> string -> string),'state,'residue,'result,'tuple> 
fun _ -> "%s %s":PrintfFormat<(int -> string -> string),'state,'residue,'result,'tuple> // failed type test on format string - good
fun _ -> "%s %s":PrintfFormat<'printer,'state,'residue,'result,(int * string)> // failed type test on 'tuple string - good
fun _ -> "%i %s":PrintfFormat<'printer,'state,'residue,'result,(int * string)> // passed - good
// nice, so can see how we can enforce type restriction on the 'printer and on the 'tuple

// so can we apply the PrintfFormat type to a printf statement (actually function)?
printfn ("%i %s":PrintfFormat<'printer,'state,'residue,'result,(int * string)>)  123 "test"
printfn ("%i %s":PrintfFormat<'printer,'state,'residue,'result,(string * string)>)  123 "test"   // nice, fails type restriction on 'tuple

// can we check for correctness of the 'result?
printfn ("%i %s":PrintfFormat<'printer,'state,'residue,unit,(int * string)>)  123 "test"    // as expected
printfn ("%i %s":PrintfFormat<'printer,'state,'residue,string,(int * string)>)  123 "test"  // nice, fails as expected