Prevent function bodies from having tail position

[shhyou]

This seems to preserve more errortrace information, but I'm not sure why.

For this program, demanding (g 0) triggers the evaluation of x at line 8, which in turn demands (f A).

(define f
  (λ (z) (z 3)))

(define g
  (λ (tt)
    ((λ (x) x)
     (f A))))

(g 0)

With appropriate source information, having e (lc/main.rkt:57) at tail position produces the following backtrace:

$ racket -lt- errortrace edit2.rkt 
do not know what function the free variable `A` stands for
  errortrace...:
   /Volumes/ramdisk/edit2.rkt:4:9: (z 3)
   /Volumes/ramdisk/edit2.rkt:8:12: x

Placing e at non-tail position improves the backtrace to:

$ racket -lt- errortrace edit2.rkt 
do not know what function the free variable `A` stands for
  errortrace...:
   /Volumes/ramdisk/edit2.rkt:4:9: (z 3)
   /Volumes/ramdisk/edit2.rkt:9:5: (f A)
   /Volumes/ramdisk/edit2.rkt:8:12: x
   /Volumes/ramdisk/edit2.rkt:8:4: ((λ (x) x) (f A))
   /Volumes/ramdisk/edit2.rkt:11:0: (g 0)

DrRacket skips the (f A) frame though.

[rfindler]

Removing tail position definitely improves error messages / stack traces (as you suggest) but it also can be problematic for space consumption.

But maybe an argument can be made that it is okay for little programs (like homework exercises, which is what this is designed for)? It still makes me nervous, tho.

Maybe a good way to go is to keep track of how many times you have gone non-tail for some given source location and then switch back to tail mode after 10 or so non-tail calls at that spot? That might lead to confusing stacktraces, tho. :(