Scheme line "values"

Years ago, when I first saw TextMate demonstrated, one of the ways it was used as a teaching tool, when teaching Ruby, was to have the current line executed, and the value it returned appended to the current line:

(2 + 3) * 4 / 5 # => 4

That is, pressing Cmd-Shift-Ctrl-E would execute the line, and update the marker.

Today, while playing around with Scheme, I came up with a neat way to do the same type of thing.

Initially, I made it so that it executed the current line, and added/updated the marker. Then, I realised I could load the file, and then execute the current line.

You can create a new bundle command, and bind it to whatever key you want, with a scope selector of source.scheme, Input of Line, Output of Replace Input.

#!/usr/bin/env bash

[[ -f "${TM_SUPPORT_PATH}/lib/bash_init.sh" ]] && . "${TM_SUPPORT_PATH}/lib/bash_init.sh"

# Evaluate the current line in our Scheme interpreter
#
# The interpreter you use should be set in the environment
# variable TM_SCHEME

# The whole file will be loaded, and the current line's value executed,
# and added to the line as a comment.

INTERPRET=${TM_SCHEME}
CMD=$(basename "$INTERPRET")

LINE=`cat /dev/stdin | sed 's/; =>.*//'`
VALUE=`echo $LINE | $INTERPRET --load $TM_FILEPATH | grep ';Value: ' | sed 's/;Value: //'`

echo -n $LINE "; =>" $VALUE

Unfortunately, trailing comments are handled as a seperate line, so getting the ruby-like behaviour of updating all of the ; => comments will have to wait for another day.

Scheme/UC Berkeley

While I’m in the “learning a new language” mode, I thought I may as well start listening to the podcasts I’d downloaded from UC Berkeley, from the well-regarded CS61A course, and the associated textbook The Structure and Interpretation of Computer Programs.

The textbook is also freely available, including in a handy PDF version, and I’m working through that too.

The language used is Scheme, which is a Lisp derivative. It actually looks a bit like Objective-C in the way functions are called. Thus you see stuff a bit like:

(+ 5 6)

(solve 4 65 3)

That is, a prefix notation is used. The function (and operators, such as +-/* are also functions) comes first, followed by the argument(s). Compare this to Objective C:

[celsiusTextField setTextColor: [NSColor blueColor]];

In Objective C, the object comes first, then the method, then the argument(s). A different structure appears where there are multiple arguments:

[object methodName: argument1 secondArgName: argument2]];

I’m still struggling to think back to my C days, but there are lots of things about every language I’ve used since python that I don’t like. Memory management, pointers, static typing, there just doesn’t seem to be any reason to have to worry about this shit.

Probably the biggest thing for me is that it is exciting me about study next year. Even if it will be learning Java, and putting up with rubbish all over again…