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underscore confusion
Mon, 2009-08-17, 18:40
Hi,
Can anyone explain this? I don't understand why the first line here
works, but the second doesn't. This seems to be an application of the
principle of maximum surprise.
List(1, 2, 3) map (List.range(1, _)) must beEqualTo(List(List(),
List(1), List(1, 2)))
List(1, 2, 3) map (List.range(1, _ + 1)) must beEqualTo(List(List(1),
List(1, 2), List(1, 2, 3)))
=> error: missing parameter type for expanded function ((x$11) =>
x$11.$plus(1))
Thanks,
Richard
Mon, 2009-08-17, 20:47
#2
Re: underscore confusion
When you use "_" as a place holder for an anonymous parameter of a function, the scope of that function is the innermost parenthesis containing it. Most of the time. For myself, I haven't been able to assert when that rule breaks. Some examples:
scala> (_)
<console>:1: error: unbound placeholder parameter
(_)
^ scala> List.range(1, _)
<console>:5: error: missing parameter type for expanded function ((x$1) => List.range(1, x$1))
List.range(1, _)
^ scala> List.range(1, _ + 1)
<console>:5: error: missing parameter type for expanded function ((x$1) => x$1.$plus(1))
List.range(1, _ + 1)
^ scala> List(1,2,3) map (_)
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3) map (_)
^ scala> List(1,2,3) map (List(_))
res3: List[List[Int]] = List(List(1), List(2), List(3)) scala> List(1,2,3).map(_)
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3).map(_)
^ scala> List(1,2,3).map((_))
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3).map((_))
^ scala> List(1,2,3).map(((_)))
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3).map(((_)))
^ So, in Richard's case, "_" alone as a parameter is treated as an anonymous parameter _outside_ that particular method, but as soon as something else is in that parameter, the scope of the function become the parameter itself. On your case, Arthur, "(2*_)" becomes "((x$1) => 2*x$1)", and the whole expression becomes, then, roughly speaking: List(1,2,3).map((x$0) => ((x$1) => 2*x$1) + 1) On Mon, Aug 17, 2009 at 2:49 PM, Arthur Peters <arthur.peters@gmail.com> wrote:
--
Daniel C. Sobral
Something I learned in academia: there are three kinds of academic reviews: review by name, review by reference and review by value.
<console>:1: error: unbound placeholder parameter
(_)
^ scala> List.range(1, _)
<console>:5: error: missing parameter type for expanded function ((x$1) => List.range(1, x$1))
List.range(1, _)
^ scala> List.range(1, _ + 1)
<console>:5: error: missing parameter type for expanded function ((x$1) => x$1.$plus(1))
List.range(1, _ + 1)
^ scala> List(1,2,3) map (_)
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3) map (_)
^ scala> List(1,2,3) map (List(_))
res3: List[List[Int]] = List(List(1), List(2), List(3)) scala> List(1,2,3).map(_)
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3).map(_)
^ scala> List(1,2,3).map((_))
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3).map((_))
^ scala> List(1,2,3).map(((_)))
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3).map(((_)))
^ So, in Richard's case, "_" alone as a parameter is treated as an anonymous parameter _outside_ that particular method, but as soon as something else is in that parameter, the scope of the function become the parameter itself. On your case, Arthur, "(2*_)" becomes "((x$1) => 2*x$1)", and the whole expression becomes, then, roughly speaking: List(1,2,3).map((x$0) => ((x$1) => 2*x$1) + 1) On Mon, Aug 17, 2009 at 2:49 PM, Arthur Peters <arthur.peters@gmail.com> wrote:
This is because "_ + 1" becomes a function and is passed to List.range(...). And since List.range does not take a function as a perameter the compiler cannot infer the anonymous functions type. That's where your compiler error comes from.
But I've run into something closely related that I thought I'd mention:List(1,2,3).map(_+1)res1: List[Int] = List(2, 3, 4)List(1,2,3).map(2*_+1)res2: List[Int] = List(3, 5, 7)
List(1,2,3).map((2*_)+1)error: missing parameter type for expanded function ((x$1) => 2.*(x$1))Why does the second line work? Shouldn't it be equivalent to the 3rd line. That being said it's nice to be able to use _ in more complex expressions, but it doesn't seem consistent.
List(1,2,3).map((2*_)+1)
^
-Arthur
On Mon, Aug 17, 2009 at 1:40 PM, Richard Rattigan <rattigan@broadinstitute.org> wrote:
Hi,
Can anyone explain this? I don't understand why the first line here works, but the second doesn't. This seems to be an application of the principle of maximum surprise.
List(1, 2, 3) map (List.range(1, _)) must beEqualTo(List(List(), List(1), List(1, 2)))
List(1, 2, 3) map (List.range(1, _ + 1)) must beEqualTo(List(List(1), List(1, 2), List(1, 2, 3)))
=> error: missing parameter type for expanded function ((x$11) => x$11.$plus(1))
Thanks,
Richard
--
Daniel C. Sobral
Something I learned in academia: there are three kinds of academic reviews: review by name, review by reference and review by value.
Tue, 2009-08-18, 05:27
#3
Re: underscore confusion
I know that (_*2) desugars to "x=>x*2". What seems odd is that "2*_+1" desugars differently from "(2*_)+1". These two expression seem like they should be in all ways equivalent because of operator precedence.
My issue is that it looks like "2*_+1" (or even "2*_+1 min 30") is treated as one expression with multiple method applications. What is the rule that defines the bounds of the placeholder binding? I've looked a bit in the spec and it isn't clear to me.
-Arthur
On Mon, Aug 17, 2009 at 3:24 PM, Daniel Sobral <dcsobral@gmail.com> wrote:
My issue is that it looks like "2*_+1" (or even "2*_+1 min 30") is treated as one expression with multiple method applications. What is the rule that defines the bounds of the placeholder binding? I've looked a bit in the spec and it isn't clear to me.
-Arthur
On Mon, Aug 17, 2009 at 3:24 PM, Daniel Sobral <dcsobral@gmail.com> wrote:
When you use "_" as a place holder for an anonymous parameter of a function, the scope of that function is the innermost parenthesis containing it. Most of the time. For myself, I haven't been able to assert when that rule breaks. Some examples: scala> (_)
<console>:1: error: unbound placeholder parameter
(_)
^ scala> List.range(1, _)
<console>:5: error: missing parameter type for expanded function ((x$1) => List.range(1, x$1))
List.range(1, _)
^ scala> List.range(1, _ + 1)
<console>:5: error: missing parameter type for expanded function ((x$1) => x$1.$plus(1))
List.range(1, _ + 1)
^ scala> List(1,2,3) map (_)
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3) map (_)
^ scala> List(1,2,3) map (List(_))
res3: List[List[Int]] = List(List(1), List(2), List(3)) scala> List(1,2,3).map(_)
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3).map(_)
^ scala> List(1,2,3).map((_))
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3).map((_))
^ scala> List(1,2,3).map(((_)))
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3).map(((_)))
^ So, in Richard's case, "_" alone as a parameter is treated as an anonymous parameter _outside_ that particular method, but as soon as something else is in that parameter, the scope of the function become the parameter itself. On your case, Arthur, "(2*_)" becomes "((x$1) => 2*x$1)", and the whole expression becomes, then, roughly speaking: List(1,2,3).map((x$0) => ((x$1) => 2*x$1) + 1) On Mon, Aug 17, 2009 at 2:49 PM, Arthur Peters <arthur.peters@gmail.com> wrote:
This is because "_ + 1" becomes a function and is passed to List.range(...). And since List.range does not take a function as a perameter the compiler cannot infer the anonymous functions type. That's where your compiler error comes from.
But I've run into something closely related that I thought I'd mention:List(1,2,3).map(_+1)res1: List[Int] = List(2, 3, 4)List(1,2,3).map(2*_+1)res2: List[Int] = List(3, 5, 7)
List(1,2,3).map((2*_)+1)error: missing parameter type for expanded function ((x$1) => 2.*(x$1))Why does the second line work? Shouldn't it be equivalent to the 3rd line. That being said it's nice to be able to use _ in more complex expressions, but it doesn't seem consistent.
List(1,2,3).map((2*_)+1)
^
-Arthur
On Mon, Aug 17, 2009 at 1:40 PM, Richard Rattigan <rattigan@broadinstitute.org> wrote:
Hi,
Can anyone explain this? I don't understand why the first line here works, but the second doesn't. This seems to be an application of the principle of maximum surprise.
List(1, 2, 3) map (List.range(1, _)) must beEqualTo(List(List(), List(1), List(1, 2)))
List(1, 2, 3) map (List.range(1, _ + 1)) must beEqualTo(List(List(1), List(1, 2), List(1, 2, 3)))
=> error: missing parameter type for expanded function ((x$11) => x$11.$plus(1))
Thanks,
Richard
--
Daniel C. Sobral
Something I learned in academia: there are three kinds of academic reviews: review by name, review by reference and review by value.
Tue, 2009-08-18, 21:57
#4
Re: underscore confusion
As I told you, the rule is that the parenthesis delimit the expression. Expressions are recursive, so you an place one inside another. There are two expressions in "(2*_)+1" and one in "2*_+1 min 30".
On Tue, Aug 18, 2009 at 1:23 AM, Arthur Peters <arthur.peters@gmail.com> wrote:
--
Daniel C. Sobral
Something I learned in academia: there are three kinds of academic reviews: review by name, review by reference and review by value.
On Tue, Aug 18, 2009 at 1:23 AM, Arthur Peters <arthur.peters@gmail.com> wrote:
I know that (_*2) desugars to "x=>x*2". What seems odd is that "2*_+1" desugars differently from "(2*_)+1". These two expression seem like they should be in all ways equivalent because of operator precedence.
My issue is that it looks like "2*_+1" (or even "2*_+1 min 30") is treated as one expression with multiple method applications. What is the rule that defines the bounds of the placeholder binding? I've looked a bit in the spec and it isn't clear to me.
-Arthur
On Mon, Aug 17, 2009 at 3:24 PM, Daniel Sobral <dcsobral@gmail.com> wrote:
When you use "_" as a place holder for an anonymous parameter of a function, the scope of that function is the innermost parenthesis containing it. Most of the time. For myself, I haven't been able to assert when that rule breaks. Some examples: scala> (_)
<console>:1: error: unbound placeholder parameter
(_)
^ scala> List.range(1, _)
<console>:5: error: missing parameter type for expanded function ((x$1) => List.range(1, x$1))
List.range(1, _)
^ scala> List.range(1, _ + 1)
<console>:5: error: missing parameter type for expanded function ((x$1) => x$1.$plus(1))
List.range(1, _ + 1)
^ scala> List(1,2,3) map (_)
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3) map (_)
^ scala> List(1,2,3) map (List(_))
res3: List[List[Int]] = List(List(1), List(2), List(3)) scala> List(1,2,3).map(_)
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3).map(_)
^ scala> List(1,2,3).map((_))
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3).map((_))
^ scala> List(1,2,3).map(((_)))
<console>:5: error: missing parameter type for expanded function ((x$1) => List(1, 2, 3).map(x$1))
List(1,2,3).map(((_)))
^ So, in Richard's case, "_" alone as a parameter is treated as an anonymous parameter _outside_ that particular method, but as soon as something else is in that parameter, the scope of the function become the parameter itself. On your case, Arthur, "(2*_)" becomes "((x$1) => 2*x$1)", and the whole expression becomes, then, roughly speaking: List(1,2,3).map((x$0) => ((x$1) => 2*x$1) + 1) On Mon, Aug 17, 2009 at 2:49 PM, Arthur Peters <arthur.peters@gmail.com> wrote:
This is because "_ + 1" becomes a function and is passed to List.range(...). And since List.range does not take a function as a perameter the compiler cannot infer the anonymous functions type. That's where your compiler error comes from.
But I've run into something closely related that I thought I'd mention:List(1,2,3).map(_+1)res1: List[Int] = List(2, 3, 4)List(1,2,3).map(2*_+1)res2: List[Int] = List(3, 5, 7)
List(1,2,3).map((2*_)+1)error: missing parameter type for expanded function ((x$1) => 2.*(x$1))Why does the second line work? Shouldn't it be equivalent to the 3rd line. That being said it's nice to be able to use _ in more complex expressions, but it doesn't seem consistent.
List(1,2,3).map((2*_)+1)
^
-Arthur
On Mon, Aug 17, 2009 at 1:40 PM, Richard Rattigan <rattigan@broadinstitute.org> wrote:
Hi,
Can anyone explain this? I don't understand why the first line here works, but the second doesn't. This seems to be an application of the principle of maximum surprise.
List(1, 2, 3) map (List.range(1, _)) must beEqualTo(List(List(), List(1), List(1, 2)))
List(1, 2, 3) map (List.range(1, _ + 1)) must beEqualTo(List(List(1), List(1, 2), List(1, 2, 3)))
=> error: missing parameter type for expanded function ((x$11) => x$11.$plus(1))
Thanks,
Richard
--
Daniel C. Sobral
Something I learned in academia: there are three kinds of academic reviews: review by name, review by reference and review by value.
--
Daniel C. Sobral
Something I learned in academia: there are three kinds of academic reviews: review by name, review by reference and review by value.
But I've run into something closely related that I thought I'd mention:
Why does the second line work? Shouldn't it be equivalent to the 3rd line. That being said it's nice to be able to use _ in more complex expressions, but it doesn't seem consistent.
-Arthur
On Mon, Aug 17, 2009 at 1:40 PM, Richard Rattigan <rattigan@broadinstitute.org> wrote: