A line fails at the prompt, and you fix it there

A function with a typo in it is applied to a list. It fails on the first item. The prompt keeps the stopped machine as a value, you look inside it, bind the name it could not find, and run it on: the list comes out whole. Nothing is retyped.

You need: the command line host and one terminal, in the directory it was built in. You have it when a prompt answers : 23.

1. Tell the prompt to keep a failure

Open a prompt:

./o.se -f o.se.osen/osen.ose

One line, in the machine’s own instruction language, since it speaks to the prompt rather than to a program. From now on a line that fails is held instead of thrown away:

> /!/inner/trap
: []

2. Write the bug

A factor, and a function that multiplies by it, with the factor’s name misspelt in the body:

> /factor : 3
/factor -> /_e
> /scale : ({ (/v, /factr) /o/mul }, ["/v"]) /o/defn
/scale -> /_e

3. Run it

> (((1, 2, 3), /scale) /o/map) /o/println
Caught exception: /factr is not bound. Did you mean /factor?
ose: halted, and held as /m. From here:
       (/m, <value>) /o/machine/store/value        bind the name it could not find, and go on
       (/m, "/name") /o/machine/fix/name           read another name instead, not run
       (/m, <value>) /o/machine/use/value          answer the step that failed with a value, and go on
       (/m, <frame>, <value>) /o/machine/return    answer an enclosing call; /o/backtrace numbers them
       (/m) /o/db                                  look at it: its stack, names and frames
       (/m) /o/machine/run                         run a copy as it stands

The map got one item in, the function read /factr, and nothing binds it. The prompt stopped the machine at that read and bound it to /m. The menu is the vocabulary for what follows.

4. Look at it

> (/m) /o/db

prints the machine whole. The parts that matter:

 /stack : (
  /v : 1,
  : "/factr",
  /error : [ 65550, "/factr is not bound. Did you mean /factor?" ],
  ...
 ),
 /env : (
  /factor : 3,
  /scale : <fn (/v)>,
  ...
  (
   /v : 1
  ),
  ()
 ),
 /pending : (
  ...
  : "/o/mul"
 ),

The stack holds the item it was on, the name it was reading and the error. The environment is every scope the line had open, the function’s own, with /v at 1, innermost. Pending is what the step had left to do: the multiply. (/m) /o/backtrace lists the open frames, the print, the map and the multiply, outermost first.

5. Bind the name, and run it on

Two repairs in one line. /o/machine/bind puts the name in every environment the machine has, so the two items to come find it; /o/machine/fix/bind supplies it to the step that already failed. Each answers a new machine and leaves /m as it was:

> /fixed : ((/m, 3, "/factr") /o/machine/bind, 3) /o/machine/fix/bind
/fixed -> /_e
> (/fixed) /o/machine/run
: [ 3, 6, 9 ]
: <machine 604 bytes>

The list printed, and the machine that printed it came back, finished.

6. A call that refused

A different kind of failure: not a name that is missing but a call that cannot answer. The menu changes with it:

> ((1, 0) /o/div) /o/println
Caught exception: out of range in /o/div
ose: halted, and held as /m. From here:
       (/m, (<args>)) /o/machine/retry/with        apply the call again, to other arguments
       (/m) /o/machine/retry                       apply it again as it was, after rebinding what it calls
       (/m, <value>) /o/machine/use/value          answer the step that failed with a value, and go on
       ...
> (/m, 2) /o/machine/use/value
: 2
: <machine 7272 bytes>

/o/machine/use/value answers the failed step with the value given and runs on: the print got 2.

What happened

A failure is kept, not reported. With the trap on, a line that fails stops where it failed and is bound to a name, whole: stack, names, open frames and what was left to do.
Every repair answers a new machine. /m is untouched, so a wrong repair costs one more line.
This is what a board does with a loop. The same machine, kept at the fault, is what arrives at the prompt in Fix a loop that failed on a board.

If something goes wrong

what you see	do
the error prints and no `held as /m` line	the trap line in step 1 was not typed in this prompt
`/m is not bound`	nothing has failed since the trap was set: run step 3
the run in step 5 answers a machine and no list	the line used `/o/machine/fix/bind` alone; the two items to come read the name again. Type the pair as shown
`/fixed -> /_e` and then an error about `/factr`	the name in the bind is spelt differently from the one in the function

Examples · Command line · Fix a loop that failed on a board