Lambda evaluation - postfix or prefix?

[Joshua Bloch]

Alex,

Along these lines I suggested * as a prefix operator, which is familiar from
function pointers in C and C++.  Doug Lea likes this.  But dot as a postfix
operator also looks pretty good.  I'm not a fan of the exclamation mark (for
this purpose).

        Josh

On Fri, Jan 22, 2010 at 6:15 PM, Alex Blewitt <alex.blewitt at gmail.com>wrote:

> The 0.1 spec that Alex Buckley sent out proposed a separate syntax for
> invocation of Lambda expressions:
>
> LambdaInvocationExpression
>  LambdaExpression LambdaInvocation
>
> LambdaInvocation:
>  '!' '(' ArgumentList_opt ')'
>  '.' '(' ArgumentList_opt ')'
>
> In other words,
>
>  #()(42)!()  // Evaluates to the value 42.
>
>  #int() fortyTwo = #()(42);
>  assert fortyTwo!() == 42;
>
> I don't know of a language which uses postfix symbols on a function as
> a determination of whether it's a first-order-function or not.
> However, I'd like to propose that we borrow 'apply' from other
> functional languages and use this as a prefix instead:
>
> LambdaInvocationExpression
>  'apply' LambdaExpression '(' ArgumentList_opt ')'
>
> This would allow the above examples to be written as
>
> apply #()(42) ()
> apply fortyTywo ()
>
> These could be nested, such that the result of one function was the
> result of another, e.g.
>
> ##int(int)(int) plus = #int(int x) #(int y) x + y
>
> apply (apply plus (1)) (1)
>
> To reduce verbosity, one could use a symbol instead of 'apply'. For
> example, we could re-use the # symbol:
>
> ##plus(1)(1)
>
> Here, the distinguishing feature is that 'plus' is not a valid type,
> and that instead it refers to a lambda expression name in the scope.
> However, it may be desirable to use a different symbol for this, so
> taking it as @pply, we could have:
>
> @@plus(1)(1)
>
> So one could think of '@' as the 'apply' or 'invocation' operator.
> Reading from left to right gives 'apply "the result of apply plus to
> one", to one'
>
> #(String s) alblue = #(String s) new Twitter("alblue").tweet(s);
>
> if (youLikeThis) {
>   @alblue("Great idea!")
> } else {
>   @alblue("That sucks!")
> }
>
>