Wednesday, February 16, 2005

lab 28 - side effects

NAME

lab 28 - side effects

DESCRIPTION

I was planning on just implementing side effects, but it turned out to be a little more complicated than I thought, and then I got sidetracked into changing the evaluator to handle more of the shell semantics.

The file lisp7.b implements setq but it is limited in that in cannot change the type of the value referenced by the symbol.

          % eval {setq a `{b c d}}
          (b c d)
          % eval {setq a `b}
          error at type mismatch
          (b c d)

Fixing this requires me re-implementing the sepxrs library to remove pick from the ADT. I'll follow up on this later.

Mixing shell and lisp syntax

The final parser in the Sussman and Steele paper The Art of the Interpreter puts dynamic scoped variables back into the evaluator. It maintains two environments, one for lexical and one for dynamic binding.

Instead of having a different dynamic environment using sexprs, I used the shell Context for the dynamic state. I also kept the shell syntax for dereferencing a dynamic variable. These and the rest of the features in this post are in lisp8.b.

          % FullForm {x := 1; echo $x; echo x}
          (Block Seq (LocalAssign x "1") (echo (Var x)) (echo x))
          % eval {x := 1; echo $x; echo x}
          ("1")
          x
          ""

The lisp evaluator in lisp8.b has some significant changes over the previous ones. I've gone back to treating shell syntax more like normal shell. Semi-colon is transformed to Seq and dollar to Var. Seq is equivalent to progn in scheme. Var dereferences a variable in the dynamic environment. Block tags a braced block which essentially quotes the whole block until explicitly eval'd.

The evaluation for values is changed. If the variable is unbound, then it evaluates to itself, like the third reference to x above.

It looks up shell commands for procedure calls. So the echo command works. We also treat normal parentheses as nested list delimiters, and use applicative order for their evaluation. This essentially does away with the ${} syntax and substitution builtins.

          % eval {x := 1 2 3 4; echo (car $x)}
          1
          ""

The language is beginning to get interesting. It looks like we have a lisp interpreter built into the handling of lists. But in fact the whole expression is converted to lisp form and evaluated. This means we can symbolically manipulate the shell commands.

          % eval {car {echo this}}
          Block

The tricky thing now is to handle more of the shell semantics including globbing, file redirection and pipes.

One assumes we will get a rather strange language at the end of it. Though I hope it will look a lot like shell with the full power of a symbolic programming language behind it.

When calling external commands, such as echo, we need to catch the case where the command doesn't exist. I want to return a form that doesn't evaluate to anything as itself.

          % eval {asdf a b}
          sh: asdf: './asdf' file does not exist
          (asdf a b)

Currently, shell prints the diagnostic that the command does not exist. I will later remove that when I move the language from a builtin to the main shell evaluator. The principle is one borrowed from computer algebra systems: Symbols evaluate to themselves if there is no other transformation.

I added support for globbing. Globbing is somewhat like macros, we are rewriting the tree before the evaluator sees it.

          % eval {echo l*}
          (lisp8.sbl lisp8.dis lisp8.b lisp7.sbl lisp7.dis lisp7.b)
          ""

I altered the FullForm for redirections and pipes but stopped short of actually implementing it. Because of lot of the code to do that is in /appl/cmd/sh/sh.y and I thought it'd be easier to move the evaluator code back into sh.y and build a new shell. I will replace the walk function in that file with eval.

Redirs are like property lists that carry with a command. The apply function needs to know about Redir. They evaluate to themselves, but when shell actually executes a command it pulls out the redirs from the expression.

          % FullForm {echo > t < x}
          (Block echo (Redir W t) (Redir R x))

CONCLUSION

I like inferno shell. It is powerful. It can be made more powerful by making it a symbolic language. I don't lose any of its current power, in particular its expressiveness for building pipelines and working with files. But I add something more. The ability for it to see itself and manipulate it's own tree of code and data.

It is useful to compare this with es shell. They took a similar idea, putting a lisp evaluator into the shell, but didn't take it to the extreme of what lisp can do. It doesn't appear the language could symbolically manipulate itself.

FUTURE

I will replace the shell evaluator with the one I have written so far and so create a prototype for new language. I can then implement the backquote, redirection and pipes.

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