An attribute enforcing usage (in F# code) of named parameters at callsite

[cloudRoutine]

Idea 7119363: An attribute enforcing usage (in F# code) of named parameters at callsite

Status : open
Submitted by Gauthier Segay on 2/19/2015 12:00:00 AM
4 votes

In some code, it's critical to have function/methods called with parameter names at call site, for readability reasons, but also for correctness.
I would like F# to enforce this using an attribute for example: []
suppose we have this code (for sake of showing the idea):
type Foo() =
member x.Bar(b, a) = (a, b)
let foo = new Foo()
foo.Bar(1.00, 0.00)
foo.Bar(a = 1.00, b = 0.00)
now suppose we have this slight change in the code (just swapped the parameter names)
type Foo() =
member x.Bar(b, a) = (a, b)
this would not produce the expected result
foo.Bar(1.00, 0.00)
With the proposed feature:
type Foo() =
[]
member x.Bar(b, a) = (a, b)
foo.Bar(1.00, 0.00) // doesn't compile, expects usage of parameter names
Another example is with FSharp.Data.SqlClient's SqlCommandProvider, which defines methods taking parameters based on their order of appearance in the sql code.

samplesql1.sql

declare @numerator float
declare @Denominator float
set @numerator = @A
set @Denominator = @b
select @numerator / @Denominator

samplesql2.sql

declare @numerator float
declare @Denominator float
set @Denominator = @b // swapped this line, didn't change anything else
set @numerator = @A
select @numerator / @Denominator

sample1.fsx

r @"..\tools\nuget\packages\FSharp.Data.SqlClient\lib\net40\FSharp.Data.SqlClient.dll"

open FSharp.Data
type divideInSql = SqlCommandProvider< "sample1.sql", YourConnectionString>
let cmd = new divideInSql()
cmd.Execute(0.0, 1.0) // work but wait bellow (don't want that to compile)
cmd.Execute(a = 0.0, b = 1.0) // works
cmd.Execute(b = 1.0, a = 0.0) // works

sample2.fsx

r @"..\tools\nuget\packages\FSharp.Data.SqlClient\lib\net40\FSharp.Data.SqlClient.dll"

open FSharp.Data
type divideInSql = SqlCommandProvider< "sample2.sql", YourConnectionString>
let cmd = new divideInSql()
cmd.Execute(0.0, 1.0) // fails divide by 0 (don't want that to compile)
cmd.Execute(a = 0.0, b = 1.0) // works

cmd.Execute(b = 1.0, a = 0.0) // works

We have seen by now that change of order in a sql file provokes havoc.
API design wise, some people might want to enforce that callsite will use named parameters to avoid those kind of misshaps.
I hope the description is clear enough.

Archived Uservoice Comments