How to use CallerMemberName, CallerFilePath and CallerLineNumber
in F# together with currying.
Solution
Let’s start with a solution. Instead of a module with functions we declare a class with static methods:
type Mod() =
static member noArg
( [<CallerLineNumber; Optional; DefaultParameterValue(0)>] line : int
) : string =
sprintf "Line %d" line
static member oneArg
( x : string,
[<CallerLineNumber; Optional; DefaultParameterValue(0)>] line : int
) : string =
sprintf "Line %d; input %s" line x
static member twoArgs
( x : string,
[<CallerLineNumber; Optional; DefaultParameterValue(0)>] line : int
) : string -> string = fun y ->
sprintf "Line %d; inputs %s, %s" line x y
static member threeArgs
( x : string,
[<CallerLineNumber; Optional; DefaultParameterValue(0)>] line : int
) : string -> string -> string = fun y z ->
sprintf "Line %d, inputs %s, %s, %s" line x y z
Hopefully it’s obvious how to create functions which take more arguments. We can test that it works as expected:
let _ = printfn "%s" (Mod.noArg ())
let _ = printfn "%s" (Mod.oneArg "hi")
let _ = printfn "%s" (Mod.twoArgs "hello" "world")
let _ = printfn "%s" (Mod.threeArgs "hello" "world" "!")
let _ = "works" |> Mod.twoArgs "piping" |> printfn "%s"
let _ = "bye" |> Mod.oneArg |> printfn "%s"
Note that the caller info attributes are recorded when a static method is converted to a function:
let f = Mod.noArg
let _ = f () |> printfn "%s" // Prints line number of the previous line
// even though `noArg` was not called there.
Also note that normal methods can be used instead of static methods. Readability can be improved by using F# optional parameters instead of C# optional parameters:
type AnotherMod() =
static member twoArgs
( x : string,
[<CallerLineNumber>] ?line : int
) : string -> string = fun y ->
sprintf "Line %d, inputs %s, %s" line.Value x y
A small drawback is that the parameter line has the type int option
instead of just int which makes this more readable alternative slightly less performant.
Non-solutions
I would rather use functions instead of methods:
let wrong1
( x : int,
[<CallerLineNumber>] line : int ) = x
let wrong2
( x : int,
[<CallerLineNumber; Optional; DefaultParameterValue(0)>] line : int ) = x
let wrong3
(x : int)
([<CallerLineNumber; Optional; DefaultParameterValue(0)>] line : int) = x
But these don’t work since the compiler is not willing to fill line parameter.
The page
Caller information
from F# language reference says that the caller info attributes work with methods,
and it doesn’t mention functions at all. So I assume it’s not possible to use functions.
Another thing is that we can’t use optional arguments with curried methods.
For example DoesNotCompileMod class
type DoesNotCompileMod() =
static member oneArg
(x : string)
([<CallerLineNumber>] ?line : int) =
sprintf "Line %d, inputs %s" line.Value x
really doesn’t compile:
[FS0440] Methods with curried arguments cannot declare ‘out’, ‘ParamArray’, ‘optional’, ‘ReflectedDefinition’, ‘byref’, ‘CallerLineNumber’, ‘CallerMemberName’, or ‘CallerFilePath’ arguments