Noncommutative null in "then - else"

[Nikolay Rychkov]

Why 

Integer?[] test = [for (i in -3 .. 3) i > 0 then i else null];

is ok, otherwise

Integer?[] test = [for (i in -3 .. 3) i > 0 then null else i];

causes errors  "Operand expression may not be an optional type: null is not assignable to Object" and "Expression must be of optional type: Null is not optional" 

[John Vasileff]

I believe, if allowed, ` i > 0 then null else i` would always evaluate to `i`, which would be confusing.

See https://github.com/ceylon/ceylon-spec/issues/88#issuecomment-3189219

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[Gavin King]

Exactly. It's a restriction we added explicitly to avoid that source
of confusion.

> https://groups.google.com/d/msgid/ceylon-users/7189F4F1-0B2C-4D41-85F7-96326C6E67AF%40vasileff.com.

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[Gavin King]

FTR, use this instead:

Integer?[] test = [for (i in -3 .. 3) if (i > 0) then i else null];

[Nikolay Rychkov]

To tell the truth it is not clear why ` i > 0 then null else i` would always evaluate to `i`

понедельник, 6 июля 2015 г., 19:44:04 UTC+3 пользователь John Vasileff написал:

I believe, if allowed, ` i > 0 then null else i` would always evaluate to `i`, which would be confusing.

See https://github.com/ceylon/ceylon-spec/issues/88#issuecomment-3189219

On Jul 6, 2015, at 12:34 PM, Nikolay Rychkov <nikolay...@gmail.com> wrote:

Why 

Integer?[] test = [for (i in -3 .. 3) i > 0 then i else null];

is ok, otherwise

Integer?[] test = [for (i in -3 .. 3) i > 0 then null else i];

causes errors  "Operand expression may not be an optional type: null is not assignable to Object" and "Expression must be of optional type: Null is not optional" 

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[Lucas Werkmeister]

`then` and `else` are separate constructs. `i > 0 then null else i` means `((i > 0) then null) else i`. First ,`(i > 0)` yields `true`. Then, `true then null` would yield null. And finally, `null else i` would yield `i`, since that’s what `else` does – it yields the right-hand side iff the left-hand side is `null`.

`i > 0 then i` should work though – no need for `else null`.

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[Gavin King]

Because " i > 0 then null else i" means "(i > 0 then null) else i"
according to the precedence rules of the then and else operators. And
the else operator returns its second argument whenever its first
argument evaluates to null.

On Mon, Jul 6, 2015 at 8:11 PM, Nikolay Rychkov

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[Nikolay Rychkov]

Thank you. Now is clear.

понедельник, 6 июля 2015 г., 21:16:33 UTC+3 пользователь Lucas Werkmeister написал: