Since I do research with F# (in particular, using F# interactive), I'd like to have switchable "print-when-in-debug" function.
I can do
let dprintfn = printfn
F# interactive says
val dprintfn : (Printf.TextWriterFormat<'a> -> 'a)
and I can use
dprintfn "myval1 = %d, other val = %A" a b
whenever I want in my scripts.
Now I'd like to define dprintfn differently, so that it would ignore all its arguments yet being syntax-compatible with printfn. How?
The closest (yet non-working) variant I have in mind is:
let dprintfn (arg: (Printf.TextWriterFormat<'a> -> 'a)) = ()
but it the following doesn't compile then dprintfn "%A" "Hello", resulting in error FS0003: This value is not a function and cannot be applied.
P.S. I currently use an alias for Debug.WriteLine(...) as work-around, but the question is still interesting for understading F#'s type system.
You can use the kprintf function, which formats a string using the standard syntax, but then calls a (lambda) function you specify to print the formatted string.
For example, the following prints the string if debug is set and otherwise does nothing:
let myprintf fmt = Printf.kprintf (fun str ->
// Output the formatted string if 'debug', otherwise do nothing
if debug then printfn "%s" str) fmt
I've been profiling my application and found that debug formatting causes significant performance issues. Debug formatting occurs on almost every string of the code, due to the nature of the application.
Obviously, this has been caused by kprintf which unconditionally formats and then passes a string to a predicate.
Finally, I came up with the following solution that may be useful for you:
let myprintf (format: Printf.StringFormat<_>) arg =
#if DEBUG
sprintf format arg
#else
String.Empty
#endif
let myprintfn (format: Printf.TextWriterFormat<_>) arg =
#if DEBUG
printfn format arg
#else
()
#endif
Usage is quite simple, and format checking works fine:
let foo1 = myprintf "foo %d bar" 5
let foo2 = myprintf "foo %f bar" 5.0
// can't accept int
let doesNotCompile1 = myprintf "foo %f bar" 5
// can't accept two arguments
let doesNotCompile2 = myprintf "foo %f bar" 5.0 10
// compiles; result type is int -> string
let bar = myprintf "foo %f %d bar" 5.0
Related
I looked up the documentation of printfn here and this is what it says:
printfn format
Print to stdout using the given format, and add a newline.
format : TextWriterFormat<'T> The formatter.
Returns: 'T The formatted result.
But if I type the following in FSI
> let v = printfn "Hello";;
Hello
val v : unit = ()
it states that v (the return value of printfn) is of type unit.
This seems inconsistent, but I guess I am missing something here, so can anybody help me out ?
It's not inconsistent. In your example, the format is of type TextWriterFormat<unit>, so the final return type is unit, because 'T is unit.
If you had written let v = printfn "Hello %s", then the type of v would be string -> unit. This is how F# provides type-safe string formatting.
I'm trying to go from:
sprintf "%3.1f" myNumber
to:
sprintf myFormatter myNumber
which is not possible
I have a situation where number precision depends on some settings, so I would like to be able to create my own formatter string.
I know it can be done with String.Format, but I am curious if there is a F# way with sprintf, or ksprinf; can it be done?
Simple answer
EDIT: Diego Esmerio on F# Slack showed me a simpler way that I honestly never thought of while working out the answer below. The trick is to use PrintfFormat directly, like as follows.
// Credit: Diego. This
let formatPrec precision =
PrintfFormat<float -> string,unit,string,string>(sprintf "%%1.%if" precision)
let x = 15.234
let a = sprintf (formatPrec 0) x
let b = sprintf (formatPrec 1) x
let c = sprintf (formatPrec 3) x
Output:
val formatPrec : precision:int -> PrintfFormat<(float -> string),unit,string,string>
val x : float = 15.234
val a : string = "15"
val b : string = "15.2"
val c : string = "15.234"
This approach is arguably much simpler than the Expr-based approach below. For both approaches, be careful with the formatting string, as it will compile just fine, but break at runtime if it is invalid.
Original answer (complex)
This isn't trivial to do, because functions like sprintf and printfn are compile-time special-case functions that turn your string-argument into a function (in this case of type float -> string).
There are some things you can do with kprintf, but it won't allow the formatting-argument to become a dynamic value, since the compiler still wants to type-check that.
However, using quotations we can build such function ourselves. The easy way is to create quotation from your expression and to change the parts we need to change.
The starting point is this:
> <# sprintf "%3.1f" #>
val it : Expr<(float -> string)> =
Let (clo1,
Call (None, PrintFormatToString,
[Coerce (NewObject (PrintfFormat`5, Value ("%3.1f")), PrintfFormat`4)]),
Lambda (arg10, Application (clo1, arg10)))
...
That may look like a whole lot of mess, but since we only need to change one tiny bit, we can do this rather simply:
open Microsoft.FSharp.Quotations // part of F#
open Microsoft.FSharp.Quotations.Patterns // part of F#
open FSharp.Quotations.Evaluator // NuGet package (with same name)
// this is the function that in turn will create a function dynamically
let withFormat format =
let expr =
match <# sprintf "%3.1f" #> with
| Let(var, expr1, expr2) ->
match expr1 with
| Call(None, methodInfo, [Coerce(NewObject(ctor, [Value _]), mprintFormat)]) ->
Expr.Let(var, Expr.Call(methodInfo, [Expr.Coerce(Expr.NewObject(ctor, [Expr.Value format]), mprintFormat)]), expr2)
| _ -> failwith "oops" // won't happen
| _ -> failwith "oops" // won't happen
expr.CompileUntyped() :?> (float -> string)
To use this, we can now simply do this:
> withFormat "%1.2f" 123.4567899112233445566;;
val it : string = "123.46"
> withFormat "%1.5f" 123.4567899112233445566;;
val it : string = "123.45679"
> withFormat "%1.12f" 123.4567899112233445566;;
val it : string = "123.456789911223"
Or like this:
> let format = "%0.4ef";;
val format : string = "%0.4ef"
> withFormat format 123.4567899112233445566;;
val it : string = "1.2346e+002f"
It doesn't matter whether the format string is now a fixed string during compile time. However, if this is used in performance sensitive area, you may want to cache the resulting functions, as recompiling an expression tree is moderately expensive.
I want to add debug printing to my project with a function having a type signature something like:
bool -> Printf.TextWriterFormat<'a> -> 'a
i.e. it should take a bool indicating whether or not we are in verbose mode, and use that to take the decision about whether to print or not.
For example, lets say dprint : bool -> Printf.TextWriterFormat<'a> -> 'a then I would like this behaviour:
> dprint true "Hello I'm %d" 52;;
Hello I'm 52
val it : unit = ()
> dprint false "Hello I'm %d" 52;;
val it : unit = ()
The idea is that a command line flag can be used to avoid control this output. I also want to avoid a runtime cost in the "not verbose" case. It is possible to define a function that works like this using kprintf:
let dprint (v: bool) (fmt: Printf.StringFormat<'a,unit>) =
let printVerbose (s: string) =
if v then System.Console.WriteLine(s)
fmt |> Printf.kprintf printVerbose
but printing/ignoring a sequence of numbers with List.iter (dprint b "%A") [1..10000] (b \in {true,false}) takes amount 1.5s for both values of b on my machine.
I came up with another method using reflection that builds an appropriately typed function to discard the formatting arguments:
let dprint (v: bool) (fmt: Printf.TextWriterFormat<'a>) : 'a =
let rec mkKn (ty: System.Type) =
if FSharpType.IsFunction(ty) then
let _, ran = FSharpType.GetFunctionElements(ty)
FSharpValue.MakeFunction(ty,(fun _ -> mkKn ran))
else
box ()
if v then
printfn fmt
else
unbox<'a> (mkKn typeof<'a>)
but here the reflection seems too expensive (even more so than that done inside the standard libraries complicated definition of printf sometimes).
I don't want to litter my code with things like:
if !Options.verbose then
printfn "Debug important value: %A" bigObject5
or closures:
dprint (fun () -> printfn "Debug important value: %A" bigObject5)
so, are there any other solutions?
I like your solution using reflection. How about caching it on the type level so that you pay the price of reflection only once per type? For example:
let rec mkKn (ty: System.Type) =
if Reflection.FSharpType.IsFunction(ty) then
let _, ran = Reflection.FSharpType.GetFunctionElements(ty)
// NOTICE: do not delay `mkKn` invocation until runtime
let f = mkKn ran
Reflection.FSharpValue.MakeFunction(ty, fun _ -> f)
else
box ()
[<Sealed>]
type Format<'T> private () =
static let instance : 'T =
unbox (mkKn typeof<'T>)
static member Instance = instance
let inline dprint verbose args =
if verbose then
printfn args
else
Format<_>.Instance
A pragmatist would just use the fast C# formatted printing machinery instead of this. I avoid Printf functions in production code because of the overhead they have, as you point out. But then F# printing definitely feels nicer to use.
My #time results for List.iter (dprint false "%A") [1..10000]:
Original version : 0.85
Original version with reflection : 0.27
The proposed version : 0.03
How about this:
/// Prints a formatted string to DebugListeners.
let inline dprintfn fmt =
Printf.ksprintf System.Diagnostics.Debug.WriteLine fmt
Then you can write:
dprintfn "%s %s" "Hello" "World!"
Debug.WriteLine(...) is marked with [<Conditional("DEBUG")>] so the F# compiler should be able to eliminate the entire statement at compile-time (though you'll have to experiment and check the compiled IL to see if it actually does.
Note that this solution only works if you don't care about changing the verbosity at run-time. If that's the case, you'll have to look for a different solution.
UPDATE : Out of curiousity, I just tried this code (it does work) and the F# 2.0 compiler doesn't compile everything away (even with optimizations on), so the speed is the same whether debugging or not. There might be other ways to get the compiler to eliminate the whole statement to fix the speed issue, but you'll just have to experiment a bit to find out.
Why not use #defines just do
let dprint (fmt: Printf.StringFormat<'a,unit>) =
#if DEBUG
let printVerbose (s: string) =
System.Console.WriteLine(s)
fmt |> Printf.kprintf printVerbose
#else
fun _ -> ()
On my machine the sample test takes 0.002s in the optimised version
In F#, how do you curry a function that accepts a variable number of parameters?
I have code like this...(the log function is just an example, the exact implementation doesn't matter)
let log (msg : string) =
printfn "%s" msg
log "Sample"
It gets called throughout the code with sprintf formatted strings, ex.
log (sprintf "Test %s took %d seconds" "foo" 2.345)
I want to curry the sprintf functionality in the log function so it looks like...
logger "Test %s took %d seconds" "foo" 2.345
I've tried something like
let logger fmt ([<ParamArray>] args) =
log (sprintf fmt args)
but I cannot figure out how to pass the ParamArray argument through to the sprintf call.
How is this done in F#?
let log (s : string) = ()
let logger fmt = Printf.kprintf log fmt
logger "%d %s" 10 "123"
logger "%d %s %b" 10 "123" true
The behaviour of printf-like functions in F# is in some way special. They take a format string, which specifies what the expected arguments are. You can use Printf.kprintf as shown by desco to define your own function that takes a format string, but you cannot change the handling of format strings.
If you want to do something like C# params (where the number of arguments is variable, but does not depend on the format string), then you can use ParamArray attribute directly on a member:
open System
type Foo =
static member Bar([<ParamArray>] arr:obj[]) =
arr |> Seq.mapi (fun i v -> printfn "[%d]: %A" i v)
Then you can call Foo.Bar with any number of arguments without format string:
Foo.Bar("hello", 1, 3.14)
This is less elegant for string formatting, but it might be useful in other situations. Unfortunatelly, it will only work with members (and not with functions defined with let)
There's information out there on how to do custom processing on a format and its parts. I want to do something a bit simpler, specifically, I want to do something to the effect of:
let writelog : string -> unit = ... // write the string to the log
let writelogf = sprintf >> writelog // write a formatted string to the log
I'm not too surprised that the compiler gets confused by this, but is there any way to get it to work?
The simplest way to define your own function that takes a formatting string like printf is to use Printf.kprintf function. The first argument of kprintf is a function that is used to display the resulting string after formatting (so you can pass it your writelog function):
let writelog (s:string) = printfn "LOG: %s" s
let writelogf fmt = Printf.kprintf writelog fmt
The fmt parameter that is passed as a second argument is the special format string. This works better than jpalmer's solution, because if you specify some additional arguments, they will be directly passed to kprintf (so the number of arguments can depend on the formatting string).
You can write:
> writelogf "Hello";;
LOG: Hello
> writelogf "Hello %d" 42;;
LOG: Hello 42
This works
> let writelog = fun (s:string) -> printfn "%s" s;;
val writelog : string -> unit
> let writelogf arg = sprintf arg >> writelog;;
val writelogf : Printf.StringFormat<('a -> string)> -> ('a -> unit)
> writelogf "hello %s" "world";;
hello world
val it : unit = ()
>
(session is from FSI)
key is in the extra argument to writelogf