Wednesday, May 21, 2008

On "I can't believe I'm praising Tcl"

Today we look at a refreshing use case for a programming language, where economy of expression in calling functions from a read-eval-print loop is prized. Raganwald recently tagged "I can't believe I'm praising TCL", in which the embedded systems author helped us understand how TCL made for a great debugger command environment, and the "pop infix languages (C/Java/Python/Ruby/you name it)" don't.

In this case the author wants to define some glue function or functions, and then in the language's interactive interpreter, call his function over and over. He's not programming; he's commanding, so the function calls need to be short and sweet, so he doesn't mind typing them for hour after hour as he thinks about the real problem, a buggy piece of embedded hardware. The author wants a command shell, where he uses his command interface to an embedded device as a kind of gdb replacement. An example session to set breakpoints, inspect memory, etc., looks like this:

$ pmem 0 stat
$ pmem 0 bkpt 0 0xbff
$ pmem 0 bkpt 1 0xa57
$ pmem 0 cmd run
$ pmem 0 stat
$ pmem 0 pc
$ pmem 0 rstack
3 return addresses
addr 0: 0x0005
addr 1: 0x05a8
addr 2: 0x0766
$ pmem 0 cmd stp
$ pmem 0 pc
The question then is whether a language you may be designing or using could support something close to this syntactic economy for calling functions.

The argument for Tcl over the pop infix language may perhaps be best summarized by this quote:
And then we have interactive shells. And in Python it’s doit("xx","yy"). And in Lisp it’s (doit "xx" "yy"), or (doit :xx :yy), or (doit xx yy) if you make it a macro. And in Ruby it’s doit :xx :yy, if you use symbols and omit parens. And that’s about as good as you can get without using your own parser as in doit "xx yy", which can suck in the (more rare) case when you do need to evaluate expressions before passing parameters, and doesn’t completely remove overhead. Also note how all these languages use (), which makes you press Shift, instead of [] which doesn’t. Ruby and Perl let you omit (), but it costs in readability. And [] is unanimously reserved for less important stuff than function calls.


First we see the emphasis is not on defining functions, on programming, but on using, on the syntax for calling, functions. The author wants an internal DSL (Domain Specific Language).

Second, it should be noted that in discussing () that Scheme lets you use [] as well as (). There's good Scheme style, where [] is reserved for the let blocks, but if you open up Dr. Scheme or Chez Scheme, define some choice Turtle graphics functions, and start typing commands like [penup] [rt 45] [pendown] [fd 100] it will work fine.

One thing the author noted is that TCL's preference for strings over variables makes bkpt a string and $bkpt a variable, whereas in the pop infix languages, it's the variables that get lexical economy and strings that need delimiters. Because of this preference, calling a Tcl command lets you pass in what look like symbols, but you treat them as strings in the command definition. Hence, for the author's use case, a chief consideration seemed to be a way to write symbolic arguments, where the command in question may take a one-of-several subcommand or option name, without lexical decoration like string delimiters or single quotes or colons. I wonder if this was really a language design goal of Tcl, because it's hard to understand the motivation for the string-vs-variable syntax any other way. For all that, enumerated types or sum types are a known language feature that meet the author's criterion. In Standard ML you could define a datatype Subcommand = bkpt | status | memset or the like, and now undecorated references like bkpt can appear as arguments.

Note if you do define your functions as Scheme macros, to address the symbol/string problem, and if you modified Scheme to accept an S-expression forest on each line (i.e. no need to delimit a top-level input line with parens), you'd have the economic expression of Tcl. I think this is worth considering in some circles where Scheme may be more familiar.


This could be a nice motify for the "language design of the day": extend the basic Scheme-like interpreter to support an extensible debugger command interface.

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