Improve warning for ignored values in sequence expressions

[forki]

// stupid things to make the sample compile
let div _ _ = 1  
let subView _ _ = [1; 2]

// elmish view
let view model dispatch =
   [
       yield! subView model dispatch
       div [] [] // <- warning
       div [] [] // <- warning
   ]

This code gives famous the famous "The result of this expression is implicitly ignored" message and suggests to bind to a let binding. This is wrong advice and not helpful for newcomers in elmish apps.

Instead we should improve the warning when we are inside of list comprehsions to something more useful.

[isaacabraham]

Because they should have yielded out the result?

[isaacabraham]

Maybe rename this to something less about elmish and more about yields in list comprehensions etc.?

[forki]

,yes they should yield. And no this is something that especially is seen in elmish view code. I think it's good to keep that reference and to think about it in that case Am 20.01.2018 12:18 schrieb "Isaac Abraham" <notifications@github.com>: Maybe rename this to something less about elmish and more about yields in list comprehensions etc.? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#4228 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AADgNHov2QY8UCyv61GMKJRPrm2UII7Xks5tMcuKgaJpZM4RlY9o> .

[isaacabraham]

But it has more applications than simply elmish. People use list comprehensions elsewhere, too.

[forki]

Another aspect here is that we often see people go over from a simple list with couple of react elements to something with an if or match. Then they need to use yield and now the whole list is basically this case here

[forki]

another common case:

let view model dispatch =
   [
       let x = true
       if x then
           div [] [] <- warning
       else
           div [] [] <- warning
   ]

interestingly the following doesn't compile:

let view model dispatch =
   [
       if true then
           div [] []
       else
           div [] []
   ]

[forki]

yet another important case:

let view model dispatch =
   [   
        div [] [
           match y with
           | 1 -> yield! subView model dispatch
           | _ -> subView model dispatch // <- warning
        ]
   ]

[forki]

see #4245