3:19 Randal talks about ambiguous grammars, which in fact Marpa can handle. But most new users are going to care about grammars that are unambiguous or nearly so. Because of its power Marpa can handle, not just any grammar you're likely to want to parse, but a lot of grammars that you probably did not feel a need to get into. Consider it like the ability of a good sports car to run smoothly at 120mph. Not something that's advisable to do on the US highways, but a sign of the kind of power and handling that can be very nice to have.
|@*Intro. This program generates clauses for the transition relation|
|from time $t$ to time $t+1$ in Conway's Game of Life, assuming that|
|all of the potentially live cells at time $t$ belong to a pattern|
|that's specified in |stdin|. The pattern is defined by one or more|
|lines representing rows of cells, where each line has `\..' in a|
|cell that's guaranteed to be dead at time~$t$, otherwise it has `\.*'.|
|The time is specified separately as a command-line parameter.|
|The Boolean variable for cell $(x,y)$ at time $t$ is named by its|
Andrew Rodland at the 2014 YAPC:NA gave a talk on Marpa. A video is available. Here are my notes on it.
The talk is very well-organized, and well-written. Andrew was a last minute fill-in, so there are glitches in the actual delivery of the material, but the material itself is near perfect.
At 4:20, it says that Marpa can modify its grammar at run time. Pedantically, this is not actually true.