scala/bug#4364. Mention null in extractor pattern #6374
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spec/08-pattern-matching.md
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| } | ||
| val z = null match { | ||
| case Pair(_, _) => "a pair" | ||
| caes _ => "not a pair" |
Ref scala/bug#4364 The specification clarification that is needed is this: What should the pattern matcher do when encountering a `null` and an extractor pattern? As it stands, SLS does not say anything about `null` for extractor pattern, which leads: - Pattern matcher will happily pass `null` into your extractor. - I can write a `null`-matching extractor MyNull. - Thus, all extractor authors must check for `null` if they would like to access the passed in argument `x`. See `Animal` example below. There are potentially two things we can do. 1. Be ok with the status quo. Make sure we handle `null` in all our extractors. 2. Not be ok with the status quo. Suppose we would want extractor pattern to be non-null values only, then that's a spec change and change in pattern matcher's behavior. It might also break existing code that handled `null` in ever so subtle way. I think Paul is suggesting option 1 in scala/bug#4364, not spec change. Given that there are no outcry of `null` handling besides scala/bug#2241, I think that's fine too. ### MyNull ```scala scala> object MyNull { | def unapply(x: AnyRef): Option[AnyRef] = | if (x eq null) Some(x) else None | } defined object MyNull scala> null match { case MyNull(_) => "yes"; case _ => "no" } res4: String = yes ``` ### Animal ```scala scala> object Animal { | def unapply(x: AnyRef): Option[AnyRef] = | if (x.toString == "Animal") Some(x) | else None | } defined object Animal scala> null match { case Animal(_) => "yes"; case _ => "no" } java.lang.NullPointerException at Animal$.unapply(<console>:13) ... 37 elided ```
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Another sample outcry: scala/bug#8787 |
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Ref scala/bug#4364 This is a general fix for scala/bug#2241 and scala/bug#8787. scala/bug#4364 raises the question of what should the pattern matcher do when encountering a `null` and an extractor pattern. As it stands, SLS does not say anything about `null` for extractor pattern, which leads: - Pattern matcher will happily pass `null` into your extractor. - I can write a `null`-matching extractor MyNull. - Thus, all extractor authors must check for `null` if they would like to access the passed in argument `x`. There are potentially two things we can do. 1. Be ok with the status quo. Clarify in SLS that all extractor authors must handle `null`. 2. Not be ok with the status quo. Change SLS and implementation such that `null` is treated as no match (empty). I've already sent option 1 as scala#6374. This implements option 2.
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Ref scala/bug#4364 This is a general fix for scala/bug#2241 and scala/bug#8787. scala/bug#4364 raises the question of what should the pattern matcher do when encountering a `null` and an extractor pattern. As it stands, SLS does not say anything about `null` for extractor pattern, which leads: - Pattern matcher will happily pass `null` into your extractor. - I can write a `null`-matching extractor MyNull. - Thus, all extractor authors must check for `null` if they would like to access the passed in argument `x`. There are potentially two things we can do. 1. Be ok with the status quo. Clarify in SLS that all extractor authors must handle `null`. 2. Not be ok with the status quo. Change SLS and implementation such that `null` is treated as no match (empty). I've already sent option 1 as scala#6374. This implements option 2.
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Closing in favor of status co-quo. |
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Ref scala/bug#4364 This is a general fix for scala/bug#2241 and scala/bug#8787. scala/bug#4364 raises the question of what should the pattern matcher do when encountering a `null` and an extractor pattern. As it stands, SLS does not say anything about `null` for extractor pattern, which leads: - Pattern matcher will happily pass `null` into your extractor. - I can write a `null`-matching extractor MyNull. - Thus, all extractor authors must check for `null` if they would like to access the passed in argument `x`. There are potentially two things we can do. 1. Be ok with the status quo. Clarify in SLS that all extractor authors must handle `null`. 2. Not be ok with the status quo. Change SLS and implementation such that `null` is treated as no match (empty). I've already sent option 1 as scala#6374. This implements option 2.
eed3si9n
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Ref scala/bug#4364 This is a general fix for scala/bug#2241 and scala/bug#8787. scala/bug#4364 raises the question of what should the pattern matcher do when encountering a `null` and an extractor pattern. As it stands, SLS does not say anything about `null` for extractor pattern, which leads: - Pattern matcher will happily pass `null` into your extractor. - I can write a `null`-matching extractor MyNull. - Thus, all extractor authors must check for `null` if they would like to access the passed in argument `x`. There are potentially two things we can do. 1. Be ok with the status quo. Clarify in SLS that all extractor authors must handle `null`. 2. Not be ok with the status quo. Change SLS and implementation such that `null` is treated as no match (empty). I've already sent option 1 as scala#6374. This implements option 2.
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Ref scala/bug#4364
The specification clarification that is needed is this: What should the pattern matcher do when encountering a
nulland an extractor pattern?As it stands, SLS does not say anything about
nullfor extractor pattern, which leads:nullinto your extractor.null-matching extractor MyNull.nullif they would like to access the passed in argumentx. SeeAnimalexample below.There are potentially two things we can do.
I think Paul is suggesting option 1 in scala/bug#4364, not spec change. Given that there are no outcry of
nullhandling besides scala/bug#2241, I think that's fine too.MyNull
Animal