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Clarify lambda positioning #10

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Merged
merged 5 commits into from
Jan 19, 2021
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Clarify lambda positioning #10

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3102baf
Clarify lambda positioning
Smaug123 Oct 26, 2020
e0d46c5
And another
Smaug123 Oct 26, 2020
6ed7ecd
Add examples from discussion
Smaug123 Nov 8, 2020
5901c69
One more example
Smaug123 Jan 19, 2021
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Merge branch 'master' into clarify-lambda
Smaug123 Jan 19, 2021
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17 changes: 16 additions & 1 deletion README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -767,7 +767,7 @@ let printVolumes x =
(convertVolumeImperialPint x)
```

The same guidelines apply for lambda expressions as function arguments. If the body of a lambda expression, the body can have another line, indented by one scope
The same guidelines apply for lambda expressions as function arguments. If the body of a lambda expression, the body can have another line, indented by one scope.

```fsharp
let printListWithOffset a list1 =
Expand All @@ -782,6 +782,21 @@ let printListWithOffset a list1 =
printfn "%d" (a + elem)
)
list1

// OK
let printListWithOffset a list1 =
list1
|> List.iter (fun elem ->
printfn "%d" (a + elem)
)

// OK
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Hi @Smaug123, what if I have something like

    innerFunc<'a, 'b>
        path'
        elementNameThatHasThisRatherLongVariableNameToForceTheWholeThingOnMultipleLines
        (foo >> bar value >> List.item a)
        shape

should this be

    innerFunc<'a, 'b> path' elementNameThatHasThisRatherLongVariableNameToForceTheWholeThingOnMultipleLines (
        foo >> bar value >> List.item a
    ) shape

Or is there a stricter rule in this case?

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I think the whole thing wants to split then, i.e. your first example:

    innerFunc<'a, 'b>
        path'
        elementNameThatHasThisRatherLongVariableNameToForceTheWholeThingOnMultipleLines
        (foo >> bar value >> List.item a)
        shape

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So when is it

A:

let printListWithOffset a list1 =
    list1
    |> List.iter (
        ((+) a)
        >> printfn "%d"
    )

and when is it

B:

 innerFunc<'a, 'b>
        path'
        elementNameThatHasThisRatherLongVariableNameToForceTheWholeThingOnMultipleLines
        (foo >> bar value >> List.item a)
        shape

I'm still sort of looking for a technical rule so to speak.
A could be because:

  • No additional arguments are passed to List.iter
  • A pipe operator is preceding List.iter

B could be because:

  • Everything before (foo >> bar value >> List.item a) already makes the whole thing too long.
  • There are additional arguments next to (foo >> bar value >> List.item a), in A there are not.

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I think it's due to the fact that the lambda is the only argument (ignoring the one we are getting piped in).

For instance:

let foo =
    List.iter (
        ((+) a)
        >> printfn "%d"
    )

Would be allowed whereas:

let foo =
    someOtherThing a b c d (
       ((+) a)
       >> printfn "%d"
    ) f g h

Would be reformatted like B

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I agree: if there is just one argument that is too long, the whole thing gets split over many lines.

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To be clear, the A example is a bit misleading. These are both right:

let printListWithOffset a list1 =
    list1
    |> List.iter (
        ((+) veryVeryVeryVeryVeryVeryVeryVeryVeryLongThing)
        >> printfn "%d"
    )

let printListWithOffset' a list1 =
    list1
    |> List.iter (((+) a) >> printfn "%d")

let printListWithOffset a list1 =
list1
|> List.iter (
((+) a)
>> printfn "%d"
)
```

However, if the body of a lambda expression is more than one line, consider factoring it out into a separate function rather than have a multi-line construct applied as a single argument to a function.
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