Multiline Parameterlists

Question

I have a member definition with a long parameterlist. For example:

member MemberExample (parameterA:int, parameterB:double, parameterC:double, ... ) =
...

Is it possible to define this member multiline? In C# it's just a line break. Using a line break in F# i received a warning:

warning FS0058: Possible incorrect indentation: this token is offside of context started at position (51:39). Try indenting this token further or using standard formatting conventions.

What's the right way in F#?

Answer 1

You can have function parameters on different lines like this:

let funcwithlongParamList p1 
    (p2: int) 
    p3 =
        p1 + p2 + p3 + 2

funcwithlongParamList 1 1 1

> 
val it : int = 5

However to improve code readability you may use:

1. Compiler type inference which helps to get rid of type annotations in function parameters

2. Function partial application if you have default values for some function parameters: http://msdn.microsoft.com/en-us/library/dd233229.aspx

3. Create a simple record type with your parameters and pass it to a function as a parameter

---

Petr

Answer 2

Hi Plepilov,

thank you for your reply.

What you've written works for me for "normal functions":

You can have function parameters on different lines like this:

let funcwithlongParamList p1 
    (p2: int) 
    p3 =
        p1 + p2 + p3 + 2

funcwithlongParamList 1 1 1

> 
val it : int = 5

In my case i want to implement a F# - library for a C# project. So i have to wrap my functional code into members - the methods for the C# project. If i define my member like you've written:

member MemberExample(parameter1,
	parameter2,
	...) =
	...

i still get the warning. Do i something wrong?

Answer 3

I think you still can use this declaration:

type SomeType() = 
    member m.someFunc a
           (b: int) (c:int) 
           (d: int) = 
        a + b + c + d + 3;

It will be compiled to method that could be consumed from C# (here is a snippet from ildasm output):

.method public instance int32  someFunc(int32 a,
                                        int32 b,
                                        int32 c,
                                        int32 d) cil managed

Also you could find this document useful:

Draft F# component design guidelines.

http://research.microsoft.com/en-us/um/cambridge/projects/fsharp/manual/fsharp-component-design-guidelines.pdf

---

Petr

Answer 4

Hi Plepilov,

thank you, that works fine.

But now i have the situation, that i have an overloaded static member.

If i write:

type SomeType() =
	static member SomeExample a b 
		(c:int) d =
			a + b + c + d

i got an error:

The method 'SomeExample' has curried arguments but has the same name as another method in this type. Methods with curried arguments cannot be overloaded. Consider using a method taking tupled arguments.

If i do not use curried arguments:

type SomeType() =
	static member SomeExample (a, b,
		(c:int), d) =
			a + b + c + d

i got the previous error:

Possible incorrect indentation: this token is offside of context started at position (24:41). Try indenting this token further or using standard formatting conventions.

Do you have any ideas how to solve this situation? I've read the component design guidlines document, but i couldn't find any help for this special situation. In both situations i use a signature file.

In the first case it looks like:

static member SomeExample: float -> float -> int -> float -> float

In the second:

static member someExample: float * float * int * float -> float

Answer 5

It seems that in this case (overloaded method) you have to use tupled arguments. As stated here: http://msdn.microsoft.com/en-us/library/dd483468.aspx

"However, overloaded methods are permitted in the language, provided that the arguments are in tuple form, not curried form."

I don't think that you could put them on different lines in this case.

---

Petr

Answer 6

You need the argument list to continue after the open parenthesis of the arguments, e.g.

type SomeType() =

    static member SomeExample(a, b,

                              (c:int), d) =

            a + b + c + d

    static member

     SomeExample2(a,

                  b,

                  (c:int),

                  d) =

            a + b + c + d

---

Brian McNamara [MSFT]

Answer 7

Hi Brian McNamara,

yes, that works. Thank you :)