Idempotency and correctness problem when using multiple if statements as list comprehension

[OkkeHendriks]

Issue created from fantomas-online

Original code

let wrong = [
        if false then 1
        if true then 2
    ]

When executed:

> let wrong = [
        if false then 1
        if true then 2
    ];;
val wrong: int list = [2]

Formatted code

let wrong = [ if false then 1; if true then 2 ]

When executed:

> let wrong = [ if false then 1; if true then 2 ];;
val wrong: int list = []

Reformatted code

let wrong =
    [ if false then
          1
          if true then 2 ]

When executed:

> let wrong =
    [ if false then
          1
          if true then 2 ];;
val wrong: int list = []

Problem description

• Fantomas broke the correctness of my code when first formatting the code.
• Fantomas was not able to produce the same code after reformatting the result.

Extra information

• The formatted result breaks by code.
• The formatted result gives compiler warnings.
• I or my company would be willing to help fix this.

Options

Fantomas master branch at 1/1/1990

Default Fantomas configuration

[nojaf]

Hello, thank you for reporting this issue.

I understand what is going on here, after the first format the second IfThenElse is merged into a Sequential of the elseExpr.
To resolve this, we could extend the current check in CodePrinter:

fantomas/src/Fantomas/CodePrinter.fs

Lines 1341 to 1343 in 6d1fd94

	if List.exists isIfThenElseWithYieldReturn xs
	|| List.forall isSynExprLambda xs then
	multilineExpression ctx

Changing List.forall isSynExprLambda to also include IfThenElse.

Are you interested in submitting a PR for this?