Short Functions

Allen Wirfs-Brock allen at wirfs-brock.com
Sun May 22 09:47:41 PDT 2011


On May 22, 2011, at 7:12 AM, Claus Reinke wrote:

> 
> For instance, block_lambdas seem to include a form of generalized jump, a callable thing that does not return to the caller (Tennent mentioned such things as 'sequels',
> so they were probably known in Algol times, but I have
> not seen them implemented, short of (delimited) continuations).

Yes, Pascal had them and so presumably did Algol 60. It was simply a goto statement whose target label was outside of the current procedure in an enclosing scope.  The Pascal compiler writers I ran with called these "bad goto's"  (bad in the sense that it took extra mechanism to make them work).  I learned the term from people who had been involved in the compilers for the Burroughs 5500 Algol machines so I presume they were present there and in Algol in general.  Too lazy right now to dig out the Algol 60 Report to check.

In Smalltalk, this is the semantics for a ^ in a block, eg [^nil]  which returns from the enclosing method invocation unlike [nil] which returns from the block invocation.  Ruby generalized it to deal with explicit control statements and break/continue style escapes.

> We just need to ensure that the callable's definition context is still active:

Yes, for example the design sketch at the very end of http://lists.squeakfoundation.org/pipermail/squeak-dev/2001-April/008237.html describes an efficient way to implement this for Smalltalk blocks.


> 
> function A() {
>   let ret = {|x| return x; }; // sequel
>   let inc = {|x| x+1 };    // function
>   let j = 0;
>   while(true) {
>       if (j > 3)
>           ret(j); // leave loop, and A
>       else
>           j = inc(j); // continue loop
>   }
> }
> 
> Calling a sequel like 'ret' is always a tail call, because it never returns to the caller, it returns elsewhere. In
> that sense, it is a generalized jump.

Yes, this exact technique is used by Smalltalk programmers

Allen
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