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Introduce a flexible scheme for declaring and checking which exceptions can be thrown. It relies on the effects as implicit capabilities pattern. The scheme is not 100% safe yet since it does not track and prevent capability capture. Nevertheless, it's already useful for declaring thrown exceptions and finding mismatches between provided and required capabilities.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,19 @@ | ||
| package scala | ||
| import language.experimental.erasedDefinitions | ||
| import annotation.implicitNotFound | ||
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| /** An ability class that allows to throw exception `E`. When used with the | ||
| * experimental.saferExceptions feature, a `throw Ex()` expression will require | ||
| * a given of class `CanThrow[Ex]` to be available. | ||
| */ | ||
| @implicitNotFound("The ability to throw exception ${E} is missing.\nThe ability can be provided by one of the following:\n - A using clause `(using CanThrow[${E}])`\n - A `canThrow` clause in a result type such as `X canThrow ${E}`\n - an enclosing `try` that catches ${E}") | ||
| erased class CanThrow[-E <: Exception] | ||
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| /** A helper type to allow syntax like | ||
| * | ||
| * def f(): T canThrow Ex | ||
| */ | ||
| infix type canThrow[R, +E <: Exception] = CanThrow[E] ?=> R | ||
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| object unsafeExceptions: | ||
| given canThrowAny: CanThrow[Exception] = ??? |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,26 @@ | ||
| -- Error: tests/neg/saferExceptions.scala:14:16 ------------------------------------------------------------------------ | ||
| 14 | case 4 => throw Exception() // error | ||
| | ^^^^^^^^^^^^^^^^^ | ||
| | The ability to throw exception Exception is missing. | ||
| | The ability can be provided by one of the following: | ||
| | - A using clause `(using CanThrow[Exception])` | ||
| | - A `canThrow` clause in a result type such as `X canThrow Exception` | ||
| | - an enclosing `try` that catches Exception | ||
| | | ||
| | The following import might fix the problem: | ||
| | | ||
| | import unsafeExceptions.canThrowAny | ||
| | | ||
| -- Error: tests/neg/saferExceptions.scala:19:48 ------------------------------------------------------------------------ | ||
| 19 | def baz(x: Int): Int canThrow Failure = bar(x) // error | ||
| | ^ | ||
| | The ability to throw exception java.io.IOException is missing. | ||
| | The ability can be provided by one of the following: | ||
| | - A using clause `(using CanThrow[java.io.IOException])` | ||
| | - A `canThrow` clause in a result type such as `X canThrow java.io.IOException` | ||
| | - an enclosing `try` that catches java.io.IOException | ||
| | | ||
| | The following import might fix the problem: | ||
| | | ||
| | import unsafeExceptions.canThrowAny | ||
| | |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,19 @@ | ||
| object test: | ||
| import language.experimental.saferExceptions | ||
| import java.io.IOException | ||
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| class Failure extends Exception | ||
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| def bar(x: Int): Int | ||
| `canThrow` Failure | ||
| `canThrow` IOException = | ||
| x match | ||
| case 1 => throw AssertionError() | ||
| case 2 => throw Failure() // ok | ||
| case 3 => throw java.io.IOException() // ok | ||
| case 4 => throw Exception() // error | ||
| case 5 => throw Throwable() // ok: Throwable is treated as unchecked | ||
| case _ => 0 | ||
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| def foo(x: Int): Int canThrow Exception = bar(x) | ||
| def baz(x: Int): Int canThrow Failure = bar(x) // error |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,15 @@ | ||
| import language.experimental.saferExceptions | ||
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| class LimitExceeded extends Exception | ||
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| val limit = 10e9 | ||
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| def f(x: Double): Double canThrow LimitExceeded = | ||
| if x < limit then x * x else throw LimitExceeded() | ||
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| @main def test(xs: Double*) = | ||
| try println(xs.map(f).sum) | ||
| catch case ex: LimitExceeded => println("too large") | ||
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,34 @@ | ||
| import language.experimental.saferExceptions | ||
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| class Fail extends Exception | ||
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| def foo(x: Int) = | ||
| try x match | ||
| case 1 => throw AssertionError() | ||
| case 2 => throw Fail() | ||
| case 3 => throw java.io.IOException() | ||
| case 4 => throw Exception() | ||
| case 5 => throw Throwable() | ||
| case _ => 0 | ||
| catch | ||
| case ex: AssertionError => 1 | ||
| case ex: Fail => 2 | ||
| case ex: java.io.IOException => 3 | ||
| case ex: Exception => 4 | ||
| case ex: Throwable => 5 | ||
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| def bar(x: Int): Int canThrow Exception = | ||
| x match | ||
| case 1 => throw AssertionError() | ||
| case 2 => throw Fail() | ||
| case 3 => throw java.io.IOException() | ||
| case 4 => throw Exception() | ||
| case _ => 0 | ||
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| @main def Test = | ||
| assert(foo(1) + foo(2) + foo(3) + foo(4) + foo(5) + foo(6) == 15) | ||
| import unsafeExceptions.canThrowAny | ||
| val x = | ||
| try bar(2) | ||
| catch case ex: Fail => 3 // OK | ||
| assert(x == 3) |