Friday, May 8, 2009

more tpsh: control flow stuff

I've been trying to find a way of hooking in proper shell control flow keywords into tpsh, without uglifing the existing code. At the moment, tpsh understands executing singular command sequences, scripts, and a queue of command sequences. It's fairly easy to modify the parsing/lexing portions to adjust the internal data structures IAW control flow keywords, the problem is how to handle execution phase.

Originally, I had in mind setting up nested data structures and doing a delayed execution: evaluate the control flow statement and modify the data, then execute the remaining commands (e.g. if CMD0; then CMD1; else CMD2; fi, would execute CMD when the statement needs to be evaluated at execution phase, then reshape the strucure so that only CMD1 or CMD2 remains, and then feed that back into the executor).

Last night, I had an interesting idea... on the case specific code generation.

Shell control flow basically amounts to very simple if, while, for, and case statements; and the more modern until and select statements. Normal execution patterns amount to using a single command sequence (e.g. cat f0 f1 f2 f3 | sort > f), or a queue of such command sequences. Why not replace that executor with a section of code, that understands how to handle those as well as control flow (etc), and then generate the desired code to execute the result.

Exempli gratia:

tpsh_cgen( ( [ 'if', 'test command' ],
             [ 'then', 'other commands' ],
             [ 'else', 'other other commands' ],
             [ 'fi' ] )

might return something like:

sub { 
    if (evaluate the 'test command' and test the exit status)
        execute the 'other commands'
    else {
        execute the 'other other commands'

and so on and so forth; so that if we call the generated code ref, we have a set of code that will execute the correct commands, whatever they may be, and with quite a lot less fuss.

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