Lambda the Ultimate

LastCalc is a very experimental web-based tool that started out as a kind of Google Calculator but with a "REPL"-style user interface that permits variable assignment.

Like Google Calculator it supports basic math, and conversions between different measurement types like miles or kilograms (relying on the Java JScience library for this functionality), all with a relatively natural language syntax.

Along the way we added the ability to define parameterized functions, and then pattern matching on datastructures like lists and maps (inspired by ML), including recursively defined functions, all in a friendly AJAX-based web user interface.

LastCalc's parser is extremely robust, doing a reasonably efficient best-first search with backtracking to interpret its input. Anything it can't interpret is ignored.

Soon people will be able to share the parameterized functions they've designed, ultimately allowing people to collaboratively "teach" LastCalc how to interpret things that it currently cannot.

The link above provides a quick walk through of the system along with a link to it so that you can try it for yourself.

Hello all!

I'm building a lisp in F# using FParsec. The language is called AML. I have a reader and an interpreter running, and I just now built a 'desugaring' function to rewrite AML code. For example, the expression a.b desugars to (access :m:b a). This is all well and good, but there is a problem. Once the desugarer outputs the transformed code text, it will be passed to the reader. As mentioned, the reader uses FParsec to create the AST. When FParsec encounters an unparsable character sequence, it propagates some nice error position information, a somewhat useful error message, and a description of the where the code text cannot be parsed. The issue is that this information will not be very meaningful to the user once the reader gets its input from the desugarer rather than the original user's code.

How do interpreters typically preserve this error information across transformed code so that the user can get error messages in terms of his original code?

Any practical or research material is nice. For run-time performance and implement-ability, I prefer a simple algorithm to a more sophisticated one. In truth, I am a language development newbie in many respects.

Thank you greatly!