recursion - TXR: writing recursive pattern matching directives -

i'm having trouble understanding how write recursive pattern matching functions in txr. below try define recursive directive recognizing file paths. know in case can represent grammar regular expression ([a-z]+\/)+[a-z]+, have more complex rule set in mind real code benefit it. causing directive fail when there forward slash?

@(define location)@\ @  (cases)@\ @/[a-z]+/@\ @  (or)@\ @/[a-z]+//@(location)@\ @  (end)@\ @(end) @(repeat) @(cases) @{location (location)} @  (output) @location valid location. @  (end) @(or) @location @  (output) @location not valid location. @  (end) @(end) @(end) 

example valid inputs:

this/is/valid this/is/also/valid a/b/c 

(of course, you're aware location matching regular language can munge regex: /[a-z]+(\/[a-z]+)*/. i'm assuming "recursion hello world" warm-up more complicated.)

the thing cases uses top bottom, short-circuited evaluation. second case cannot match since first case matches prefix. it's not regex branch operator order of subexpressions doesn't matter.

if swap 2 cases, sample works me.

what works (in particular case) changing cases some. some directive doesn't stop @ first match.

using some not cure-all sort of cases ordering problem because need short circuit around case terminate recursion (for instance, avoid left-recursing when condition hit) or avoid degenerate performance (exponential time). i'm reminded of university professor's joke: "you've heard of divide , conquer; multiply , surrender".

some has property later clauses "see" bindings earlier matching clauses. might interfere being solution cases-ordering problem. say, later clause might fail match due variable clash. :resolve feature of some might helpful in situation or might not.