> This implies that this mechanism is limited to the kinds of things that
> can be represented using mapped files.
The statement is true, but not for the reasons you imply (or that I infer
you imply).
The read-only mechanism works by setting a flag in the name's header. All
J headers share the same format [1]. Therefore any J name of any class
can theoretically be marked as read-only (i.e. not just mapped nouns, nor
just nouns).
For an example, here's an excerpt from a J601 session:
plus=:+
15!:6{.;:'plus'
22098584
require 'jmf'
1 readonly_jmf_ 'plus'
plus =: -
|read-only data
| plus =:-
Note that this has nothing to do with mapped files; readonly_jmf_ is just
a utility to set that flag in a name's header. This works by getting a
refernce to the name (using 15!:6 ), reading in the flags field of the
underlying structure, changing the readonly flag, and writing the field
back.
You'll notice two things: one, I said "theoretically"; two, the excerpt
from is J601. That's because J602 introduced a frustrating change. It is
no longer possible to get a reference to a non-noun name using 15!:6 .
Concretely, the difference is:
9!:14 '' NB. J601
j601/2006-11-17/17:05
aNoun =: 'noun'
aVerb =: +/ % #
15!:6 <'aNoun'
22098656
15!:6 <'aVerb'
22098632
versus:
9!:14 '' NB. J602
j602/2008-03-03/16:45
aNoun =: 'noun'
aVerb =: +/ % #
15!:6 <'aNoun'
26038640
15!:6 <'aVerb'
|domain error
| 15!:6<'aVerb'
|[-7]
Though J602 is fairly new, this change has already thwarted me on several
occasions, and I wish it would be rolled back. What was its rationale?
-Dan
[1] http://www.jsoftware.com/jwiki/Essays/Incunabulum
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