[Lift] Re: Newbie: zero parameter functions, bind call-by-name function curiosity
On Oct 27, 8:50 pm, Strom wrote:
> Yes Ross, I did mean () => processForm. Thanks for the clarification.
> The one thing in moving from Java to Scala is that there are a lot of
> things that look similar that aren't, as you've pointed out. Newbie
> error caught ;)
>
> Thanks Marius, I made that assumption, but it still bugs me that the
> scala interpreter evaluates both the "value" and the "function" when I
> use:
>
> scala> def testFunc() : Int = 3
>
> scala> testFunc
> Int = 3
>
> scala>testFunc()
> Int = 3
Right ... and this is correct. Because there is no context the
interpreter (and compiler) assume that you mean a function-
application. But try this:
scala> testFunc _
res5: () => Int =
the _ here means eta expansion. A method is converted into a function.
SLS 6.25.5
But context matters:
scala> def testFunc(): Int = 3
testFunc: ()Int
scala> def other(f: () => Int) = f()
other: (() => Int)Int
scala> other(testFunc)
res0: Int = 3
scala> other(testFunc())
:7: error: type mismatch;
found : Int
required: () => Int
other(testFunc())
^
In the above example textFunc and testFunc() means two different
things
>
> Strom
>
> On Oct 27, 7:19 am, Ross Mellgren wrote:
>
> > Did you mean () => processForm by chance? () -> processForm is a pair
> > of () and processForm's result.
>
> > -Ross
>
> > On Oct 27, 2009, at 2:52 AM, Strom wrote:
>
> > > I'm not sure if this is a scala or a lift related question, but when I
> > > try to bind a submit button, I'm having some confusion on using zero
> > > parameter functions as the "() -> Any" parameter in the submit
> > > signature.
>
> > > The signature for the method in question is
> > > SHtml.submit(value : String, func : () => Any, attrs : (String,
> > > String)
> > > *)
>
> > > 1. My first question is about the following method definitions
> > > def processForm () : Unit = {...}
> > > vs
> > > def processForm = {...} //no parentheses after method declaration
>
> > > Why can I call "processForm" as well as "processForm()" for the first
> > > example, but only "processForm" for the second example. If there is
> > > scala/lift documentation on this simple question?
>
> > > 2. Why do I get a compiler error when I use the following code:
> > > bind( ...
> > > "submit" -> SHtml.submit( "Process Form", processForm() ) /*type
> > > mismatch error*/
> > > )
> > > vs
> > > bind( ...
> > > "submit" -> SHtml.submit( "Process Form", processForm ) /*compiles
> > > successfully*/
> > > )
>
> > > It's my impression that the above bind calls are equivalent given the
> > > way the scala compiler behaves when I call "processForm" or
> > > "processForm()".
>
> > > Also, I wasn't even sure the method itself would allow the code to
> > > compile. I thought I'd have to do "() -> processForm" or "() ->
> > > processForm()".
>
> > > The error I was getting was this:
> > > error type mismatch
> > > found : Unit
> > > required : () -> Any
>
> > > Can someone please clarify? Thanks so much.
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[Lift] Re: Newbie: zero parameter functions, bind call-by-name function curiosity
Yes Ross, I did mean () => processForm. Thanks for the clarification.
The one thing in moving from Java to Scala is that there are a lot of
things that look similar that aren't, as you've pointed out. Newbie
error caught ;)
Thanks Marius, I made that assumption, but it still bugs me that the
scala interpreter evaluates both the "value" and the "function" when I
use:
scala> def testFunc() : Int = 3
scala> testFunc
Int = 3
scala>testFunc()
Int = 3
Strom
On Oct 27, 7:19 am, Ross Mellgren wrote:
> Did you mean () => processForm by chance? () -> processForm is a pair
> of () and processForm's result.
>
> -Ross
>
> On Oct 27, 2009, at 2:52 AM, Strom wrote:
>
>
>
> > I'm not sure if this is a scala or a lift related question, but when I
> > try to bind a submit button, I'm having some confusion on using zero
> > parameter functions as the "() -> Any" parameter in the submit
> > signature.
>
> > The signature for the method in question is
> > SHtml.submit(value : String, func : () => Any, attrs : (String,
> > String)
> > *)
>
> > 1. My first question is about the following method definitions
> > def processForm () : Unit = {...}
> > vs
> > def processForm = {...} //no parentheses after method declaration
>
> > Why can I call "processForm" as well as "processForm()" for the first
> > example, but only "processForm" for the second example. If there is
> > scala/lift documentation on this simple question?
>
> > 2. Why do I get a compiler error when I use the following code:
> > bind( ...
> > "submit" -> SHtml.submit( "Process Form", processForm() ) /*type
> > mismatch error*/
> > )
> > vs
> > bind( ...
> > "submit" -> SHtml.submit( "Process Form", processForm ) /*compiles
> > successfully*/
> > )
>
> > It's my impression that the above bind calls are equivalent given the
> > way the scala compiler behaves when I call "processForm" or
> > "processForm()".
>
> > Also, I wasn't even sure the method itself would allow the code to
> > compile. I thought I'd have to do "() -> processForm" or "() ->
> > processForm()".
>
> > The error I was getting was this:
> > error type mismatch
> > found : Unit
> > required : () -> Any
>
> > Can someone please clarify? Thanks so much.
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[Lift] Re: Newbie: zero parameter functions, bind call-by-name function curiosity
Did you mean () => processForm by chance? () -> processForm is a pair
of () and processForm's result.
-Ross
On Oct 27, 2009, at 2:52 AM, Strom wrote:
>
> I'm not sure if this is a scala or a lift related question, but when I
> try to bind a submit button, I'm having some confusion on using zero
> parameter functions as the "() -> Any" parameter in the submit
> signature.
>
> The signature for the method in question is
> SHtml.submit(value : String, func : () => Any, attrs : (String,
> String)
> *)
>
> 1. My first question is about the following method definitions
> def processForm () : Unit = {...}
> vs
> def processForm = {...} //no parentheses after method declaration
>
> Why can I call "processForm" as well as "processForm()" for the first
> example, but only "processForm" for the second example. If there is
> scala/lift documentation on this simple question?
>
> 2. Why do I get a compiler error when I use the following code:
> bind( ...
> "submit" -> SHtml.submit( "Process Form", processForm() ) /*type
> mismatch error*/
> )
> vs
> bind( ...
> "submit" -> SHtml.submit( "Process Form", processForm ) /*compiles
> successfully*/
> )
>
> It's my impression that the above bind calls are equivalent given the
> way the scala compiler behaves when I call "processForm" or
> "processForm()".
>
> Also, I wasn't even sure the method itself would allow the code to
> compile. I thought I'd have to do "() -> processForm" or "() ->
> processForm()".
>
>
> The error I was getting was this:
> error type mismatch
> found : Unit
> required : () -> Any
>
> Can someone please clarify? Thanks so much.
>
> >
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[Lift] Re: Newbie: zero parameter functions, bind call-by-name function curiosity
On Oct 27, 8:52 am, Strom wrote:
> I'm not sure if this is a scala or a lift related question, but when I
> try to bind a submit button, I'm having some confusion on using zero
> parameter functions as the "() -> Any" parameter in the submit
> signature.
>
> The signature for the method in question is
> SHtml.submit(value : String, func : () => Any, attrs : (String, String)
> *)
>
> 1. My first question is about the following method definitions
> def processForm () : Unit = {...}
> vs
> def processForm = {...} //no parentheses after method declaration
>
> Why can I call "processForm" as well as "processForm()" for the first
> example, but only "processForm" for the second example. If there is
> scala/lift documentation on this simple question?
Because compiler already knows that processForm is a zero argument
function and accepts processForm call. In the second example you just
can't call a call-by-name using () as it would contradict its very
definition.
>
> 2. Why do I get a compiler error when I use the following code:
> bind( ...
> "submit" -> SHtml.submit( "Process Form", processForm() ) /*type
> mismatch error*/
> )
> vs
> bind( ...
> "submit" -> SHtml.submit( "Process Form", processForm ) /*compiles
> successfully*/
> )
processForm() is a value not a function. It is the value returned by
processForm function while processForm is the function itself.
>
> It's my impression that the above bind calls are equivalent given the
> way the scala compiler behaves when I call "processForm" or
> "processForm()".
>
> Also, I wasn't even sure the method itself would allow the code to
> compile. I thought I'd have to do "() -> processForm" or "() ->
> processForm()".
>
> The error I was getting was this:
> error type mismatch
> found : Unit
> required : () -> Any
>
> Can someone please clarify? Thanks so much.
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