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Comment by DonaldFisk | Hacker News Reader

I found none of these particularly difficult. (I've never attempted 3. or 5. though.) Functional values, however, are difficult. They work effortlessly in interpreted code, but in compiled code you're faced with the upwards funarg problem: https://en.wikipedia.org/wiki/Funarg_problem#Upwards_funarg_...

A solution is needed if you want lazy evaluation.

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> They work effortlessly in interpreted code, but in compiled code you're faced with the upwards funarg problem

No it's not about interpreted or compiled. It's about using the native stack or a heap for stack frames. Compiled code and interpreters can both use either, so that's orthogonal.

Yes, to solve the upwards funarg problem you can't rely on a single stack. The problem is that using the heap for everything instead comes with a heavy computational cost, and using the heap only when you need it means you need to identify those circumstances, which isn't trivial.

How did you do lambdas and lexical scoping without 'functional values'?

Presumably by implementing only the simpler "downwards" funargs: the ability to pass functions into a function scope but never return them.

That's explained in https://en.wikipedia.org/wiki/Funarg_problem#Downwards_funar...

In addition to return address and dynamic link, you need to store a static link in each stack frame.

5 is do-able by an undergrad, though the first time you encounter the concept it can be difficult to grapple how you would implement it.

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