Allen's lambda syntax proposal

Brendan Eich brendan at
Thu Dec 4 22:27:40 PST 2008

On Dec 4, 2008, at 10:12 PM, Brendan Eich wrote:

> On Dec 4, 2008, at 7:45 PM, Michael Day wrote:
>> Hi Brendan,
>>> The main contention about lambdas ignoring syntax is whether the  
>>> completion-value creates a hazard that needs to be treated  
>>> somehow, or even judged as fatal to the proposal.
>> Completion value, like the last thing to be evaluated in the  
>> lambda? What exactly is the nature of the hazard?
> Functional programming favors using completion values -- function  
> call results propagate back up naturally this way. Chaining,  
> filters, etc. all work the way you want. Here's the Y combinator:
> const fact = lambda(proc) {
>    return lambda (n) { (n <= 1) ? 1 : n * proc(n-1); }

D'oh -- I wrote return incorrectly there. That means, by Tennent's  
Correspondence Principle, that if the above were embedded in a  
function, the return would force control flow to return from the  
function as well as the outer lambda (the one assigned to const fact),  
and the return value would be the inner lambda.

This is the other hazard with lambdas. The program equivalences  
Tennent's Correspondence Principle enables are  good for refactoring,  
but bad for thinkos like the above.

(Honest, I didn't do it on purpose!)


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