gettimothy wrote on Fri, 19 Jan 2024 10:27:45 -0500
Thanks for the replies on how to begin to think about a Context. vs Environments.
Next up in the SICP study are the terms. applicative order evaluation where the operands are evaluated first and normal order evaluation, the operands are not evaluated until their values are needed.
We have both in Smalltalk using blocks. Evaluated arguments:
z foo: (3+4) bar: (z/2)
Non evaluated arguments (#foo:bar: must explicitly send #value to the arguments when it wants them to finally be evaluated):
z foo: [3+4] bar: [z/2]
Note that this is something that has changed quite a bit from Smalltalk-72 to -76 to -80. In Smalltalk-72 the code would look like
z foo (3+4) bar (z/2)
in both cases and you would have to look at the code in z's class to tell which one you are dealing with. The colon character would fetch evaluated arguments while the open colon character the unevaluated arguments. As the language evolved we passed most of the control for this from the receiver to the sender, though the receiver still has to do the right thing.
I vaguely remember Eliot relying on Squeak strict Right to Left (or is it Left to Right) evaluation rule in an email from some years ago.
We have had both left-to-right and right-to-left Smalltalks. It is a bit like the choice between starting array indexes at 0 or at 1. One option is more newbie friendly while the other makes more sense at the implementation level.
People read code from left to right so evaluating the arguments in that order leads to fewer surprises. But doing it right to left has the advantage of leaving the first argument, which is the receiver to which we want to send the message, always at the top of the stack. Evaluating left to right leaves the receiver buried under the arguments and forces you to have separate "send", "send1", "send2", etc bytecodes. A right to left Smalltalk can get by with a single "send" bytecode.
Squeak evaluates left to right and uses 1 as the first array index. Self evaluates right to left and uses 0 as the first array index.
But note that I consider code that gives different results when evaluated in different orders badly written, if not exactly buggy. Such code will be a problem if you want to have a more parallel Smalltalk.
-- Jecel