Re: [Lift] Safe handling of long params
Ross Mellgren writes:
> In your case, asLong is:
>
> def asLong(s: String): Box[Long]
>
> So if you used map:
>
> Full("1234").map(asLong) // result is Full(Full(1234)), because the
> result of asLong went into the Full that map creates
>
> Conversely, flatMap removes a level of structure:
>
> Full("1234").flatMap(asLong) // result is Full(1234)
>
> Hope that helps,
It does, thanks!
--
Jim Wise
jw...@draga.com
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Re: [Lift] Safe handling of long params
They aren't. Here are the type signatures for map and flatMap, from
Box (the type on Seq is the same, but not specialized to the Box
return type):
def map[B](f: A => B): Box[B]
def flatMap[B](f: A => Box[B]): Box[B]
The key is the return type of "f" -- in map, it always returns what
goes in the Full box, whereas in flatMap it returns any type of box
(which could be empty, failure, etc).
To illustrate better, here are some examples:
def f1(i: Int): String = i.toString
Full(1).map(f1) // result is Full("1")
Full(2).map(f1) // result is Full("2")
Empty.map(f1) // result is Empty (f1 never applied)
Failure(...).map(f1) // result is Failure (f1 never applied)
def f2(i: Int): Box[String] = if (i % 2 == 0) Full(i.toString) else
Failure("number is not even")
Full(1).flatMap(f2) // result is Failure("number is not even", ...)
Full(2).flatMap(f2) // result is Full("2")
Empty.flatMap(f2) // result is Empty (f2 never applied)
Failure(...).flatMap(f2) // result is Failure (f2 never applied)
As you can see, functions passed to map can never "fail", that is,
they can never cause a Box to become empty. The result of a map is as
empty as the input to it.
Conversely, with flatMap the function passed in can "fail", making a
non-Full box.
So, flatMap is used for chaining together computations (functions)
that can fail, whereas map is used for computations that can't fail
(barring exceptions)
This ties in with the collection-like view of a Box, which can be
views as a single-element collection (Full) or empty collection
(Empty, Failure) where the empty collection might have some extra
information about why the collection is empty.
In your case, asLong is:
def asLong(s: String): Box[Long]
So if you used map:
Full("1234").map(asLong) // result is Full(Full(1234)), because the
result of asLong went into the Full that map creates
Conversely, flatMap removes a level of structure:
Full("1234").flatMap(asLong) // result is Full(1234)
Hope that helps,
-Ross
On Jan 5, 2010, at 10:45 AM, Jim Wise wrote:
>>>
>>> I can answer b) -- S.param("image").flatMap(asLong)
>>>
>>> asLong comes from BasicTypesHelpers.
>>>
>>> -Ross
>
> One more quick question about this, more of a Scala question really:
> I've noticed both here and in the examples that there is a tendency to
> use flatMap() instead of map(). I understand the differences between
> the two, but am not sure why flatMap is preferred when there aren't
> Lists involved. Any reason? Aren't the two interchangeable in this
> case?
>
> Thanks,
> --
> Jim Wise
> jw...@draga.com
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Re: [Lift] Safe handling of long params
map would result in a Box[Box[Long]]
-
Jim Wise wrote:
>>
>> I can answer b) -- S.param("image").flatMap(asLong)
>>
>> asLong comes from BasicTypesHelpers.
>>
>> -Ross
One more quick question about this, more of a Scala question really:
I've noticed both here and in the examples that there is a tendency to
use flatMap() instead of map(). I understand the differences between
the two, but am not sure why flatMap is preferred when there aren't
Lists involved. Any reason? Aren't the two interchangeable in this
case?
Thanks,
--
Jim Wise
jw...@draga.com
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Re: [Lift] Safe handling of long params
>>
>> I can answer b) -- S.param("image").flatMap(asLong)
>>
>> asLong comes from BasicTypesHelpers.
>>
>> -Ross
One more quick question about this, more of a Scala question really:
I've noticed both here and in the examples that there is a tendency to
use flatMap() instead of map(). I understand the differences between
the two, but am not sure why flatMap is preferred when there aren't
Lists involved. Any reason? Aren't the two interchangeable in this
case?
Thanks,
--
Jim Wise
jw...@draga.com
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Re: [Lift] Safe handling of long params
Aha!
Thanks,
Sent from my iPhone
On Jan 4, 2010, at 16:45, Ross Mellgren wrote:
>
> I can answer b) -- S.param("image").flatMap(asLong)
>
> asLong comes from BasicTypesHelpers.
>
> -Ross
>
> On Jan 4, 2010, at 4:42 PM, Jim Wise wrote:
>
>>
>> So, in my endless retweaking of this code, getting better (and
>> learning
>> more lift) on each pass, I've got a rewrite rule like this
>>
>>case RewriteRequest(ParsePath("image" :: image :: Nil, _, _,
>> _), _, _) =>
>> RewriteResponse("viewImage" :: Nil, Map("image" -> image))
>>
>> which is (eventually) handled by a snippet like this:
>>
>> def doShowOne(in: NodeSeq): NodeSeq =
>> S.param("image").map(_.toLong) match {
>> case Full(selected) =>
>> ImageInfo.findByKey(selected) match {
>> case Full(i) =>
>> bind(...)
>> case _ =>
>> S.error("No such image!")
>> }
>> case _ =>
>> S.redirectTo("/images")
>> }
>>
>> Which works well if the referenced path component is the id of an
>> existing image, or of a nonexistent image -- and throws an exception
>> if
>> the last path component can't be parsed as a long.
>>
>> I can catch the exception, of course, but I kinda feel there must
>> be a
>> cleaner way to do this -- either by not matching in the rewriting PF
>> if
>> the positional paramter "image" is not parseable as a long, or by
>> making
>> this another layer of casing in the snippet.
>>
>> So, two questions:
>>
>> a.) is there an idiom for this?
>>
>> b.) is there a good way to test if a String (or better, a Box
>> [String])
>> is convertable toLong without a try/catch?
>>
>> Thanks again,
>> --
>>Jim Wise
>>jw...@draga.com
>
> --
>
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Re: [Lift] Safe handling of long params
Ah right yeah - I thought it was implicit :)
On 4 Jan 2010, at 22:09, Ross Mellgren wrote:
> Nope, it's not a implicit extension of String or anything, it's just a
> plain method def asLong(s: String): Box[Long] in BasicTypesHelpers
>
> -Ross
>
> On Jan 4, 2010, at 5:08 PM, Timothy Perrett wrote:
>
>> Shouldnt that be:
>>
>> S.param("image").flatMap(_.asLong)
>>
>> Cheers, Tim
>>
>> On 4 Jan 2010, at 21:45, Ross Mellgren wrote:
>>
>>> S.param("image").flatMap(asLong)
>>
>> --
>>
>> You received this message because you are subscribed to the Google
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>> .
>>
>>
>
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>
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Re: [Lift] Safe handling of long params
Nope, it's not a implicit extension of String or anything, it's just a
plain method def asLong(s: String): Box[Long] in BasicTypesHelpers
-Ross
On Jan 4, 2010, at 5:08 PM, Timothy Perrett wrote:
> Shouldnt that be:
>
> S.param("image").flatMap(_.asLong)
>
> Cheers, Tim
>
> On 4 Jan 2010, at 21:45, Ross Mellgren wrote:
>
>> S.param("image").flatMap(asLong)
>
> --
>
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>
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Re: [Lift] Safe handling of long params
Shouldnt that be:
S.param("image").flatMap(_.asLong)
Cheers, Tim
On 4 Jan 2010, at 21:45, Ross Mellgren wrote:
> S.param("image").flatMap(asLong)
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Re: [Lift] Safe handling of long params
I can answer b) -- S.param("image").flatMap(asLong)
asLong comes from BasicTypesHelpers.
-Ross
On Jan 4, 2010, at 4:42 PM, Jim Wise wrote:
>
> So, in my endless retweaking of this code, getting better (and
> learning
> more lift) on each pass, I've got a rewrite rule like this
>
> case RewriteRequest(ParsePath("image" :: image :: Nil, _, _,
> _), _, _) =>
> RewriteResponse("viewImage" :: Nil, Map("image" -> image))
>
> which is (eventually) handled by a snippet like this:
>
> def doShowOne(in: NodeSeq): NodeSeq =
>S.param("image").map(_.toLong) match {
> case Full(selected) =>
>ImageInfo.findByKey(selected) match {
> case Full(i) =>
>bind(...)
> case _ =>
>S.error("No such image!")
>}
> case _ =>
>S.redirectTo("/images")
>}
>
> Which works well if the referenced path component is the id of an
> existing image, or of a nonexistent image -- and throws an exception
> if
> the last path component can't be parsed as a long.
>
> I can catch the exception, of course, but I kinda feel there must be a
> cleaner way to do this -- either by not matching in the rewriting PF
> if
> the positional paramter "image" is not parseable as a long, or by
> making
> this another layer of casing in the snippet.
>
> So, two questions:
>
> a.) is there an idiom for this?
>
> b.) is there a good way to test if a String (or better, a Box
> [String])
> is convertable toLong without a try/catch?
>
> Thanks again,
> --
> Jim Wise
> jw...@draga.com
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[Lift] Safe handling of long params
So, in my endless retweaking of this code, getting better (and learning
more lift) on each pass, I've got a rewrite rule like this
case RewriteRequest(ParsePath("image" :: image :: Nil, _, _, _), _, _)
=>
RewriteResponse("viewImage" :: Nil, Map("image" -> image))
which is (eventually) handled by a snippet like this:
def doShowOne(in: NodeSeq): NodeSeq =
S.param("image").map(_.toLong) match {
case Full(selected) =>
ImageInfo.findByKey(selected) match {
case Full(i) =>
bind(...)
case _ =>
S.error("No such image!")
}
case _ =>
S.redirectTo("/images")
}
Which works well if the referenced path component is the id of an
existing image, or of a nonexistent image -- and throws an exception if
the last path component can't be parsed as a long.
I can catch the exception, of course, but I kinda feel there must be a
cleaner way to do this -- either by not matching in the rewriting PF if
the positional paramter "image" is not parseable as a long, or by making
this another layer of casing in the snippet.
So, two questions:
a.) is there an idiom for this?
b.) is there a good way to test if a String (or better, a Box[String])
is convertable toLong without a try/catch?
Thanks again,
--
Jim Wise
jw...@draga.com
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