Improve error reporting: warning if returning ignore

[smoothdeveloper]

When beginning with F# (or when exhausted), it can happen to be writing code which mistakenly returns ignore such as:

[<Test>] let ``my test`` () =
  // some code

  ignore

I think this is in many case unintended and warrants a warning.

Did you intend to return 'ignore' or to use it to discard previous expression?
#1103

[forki]

What's wrong with it? The fact that we don't ignore anything? And what is
the expected error message?
On Apr 25, 2016 4:59 PM, "Gauthier Segay" notifications@github.com wrote:

When beginning with F# (or when exhausted), it can happen to be writing
code which mistakenly returns ignore such as:

[] let my test () =
// some code

ignore

I think this is in many case unintended and warrants a warning.

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#1113

[smoothdeveloper]

@forki, added the error message.

What is wrong is that the function signature in this case turns into unit -> ('a -> unit) instead of unit -> unit.

Try to put a NUnit test such as this one and to run it, I think I've seen this mistake few times when someone is not used to the pipe operator.

Of course if people think it is a stupid warning we can forget about it 😄

[isaacabraham]

so in other words you meant to return () or just ignore()?

I guess this is the same as returning a function in any function - is what we're saying is "should we return a warning any time you return a function"? Can't be. Is ignore a special case though? Would you ever normally want to return it? Maybe not.

[smoothdeveloper]

@isaacabraham intent is to just make that happen for ignore from FSharp.Core, it is OK to return a function otherwise (or to discard the warning if I really want to return ignore).

[ReedCopsey]

@smoothdeveloper I wonder if this would be better off as part of FSharpLint instead?

[forki]

Yes this one looks too specific to "ignore" - I don't think we can bake that easily into the compiler. I mean there is probably a way, but it feels like this one should go into linter.

[forki]

Or maybe it can go into nunit. The runner could complain if it finds tests with wrong signature

[smoothdeveloper]

@ReedCopsey / @forki makes sense, I'll see if I can make a contribution to FSharpLint thanks!