I encountered a situation that need to break a loop while invoking
Scheme code in LilyPond. Typically LilyPond offers the `for-each`
function which simulates the for-loop in most programming languages.
However, the `break` functionality isn't explicitly offered. There seems
to be a function named `break` in Scheme but it is not what `break` in
most common programming languages mean. I need to find a proper way to
implement the `break`.
The purpose of my code is to get a value or an entry handle from a
multi-layer `hash-table` in Scheme. Here is a simulation of the process
in Python:
```py
defdeepGet( dic, keys, default= None):
target=dic
forkeyinkeys:
ifkeyintarget: target=target[ key]
else: returndefault
returntarget
```
However, in Scheme, returning a value inside a loop is troublesome. I
must first break the loop and then return the value. So what is the
correct way of doing this?
My current code is as follows:
```ly
#(define* ((_hash-table-deep-ref getter) h keys #:optional (fallback #f))
(let* ((target h))
(for-each
(lambda (key)
(set! target (getter target key)))
keys)
target))
#(define hash-deep-ref (_hash-table-deep-ref hash-ref))
#(define hashq-deep-ref (_hash-table-deep-ref hashq-ref))
#(define hashv-deep-ref (_hash-table-deep-ref hashv-ref))
```
The code works fine only if all the sub-tables are present for the given
keys. If any of the sub-table is not present for one key in the middle
of the key list, this function would fail. My ideal case is that when
such a case happens, the fallback value should be returned. Also I want
to create a version of the function getting the hash entry handle from a
multi-layer hash table.
