CRUD
CRUD annotations are used in services to constrain the functionality of fields and boxes in an INTERFACE-statement. This allows you to minimize the functionality for your users, to design for easy learning.
Each CRUD annotation comes right after a term, so we can always refer to "the term" to which a CRUD annotation belongs. A CRUD annotation constrains the things your user can do with the target atoms and the pairs of its term.
The CRUD-annotation specifies Create, Read, Update, and Delete rights for the term it follows. Capital = allowed, Non-capital = not allowed. CRUD is the default, so if you specify nothing, everything is allowed. The following service definition illustrates this.
INTERFACE Overview : "_SESSION" cRud
BOX <TABS>
[ Students : V[SESSION*Student] cRuD
BOX <TABLE>
[ "Student" : I[Student] cRud
, "Enrolled for" : isEnrolledFor cRUD
, "Course" : takes CRUD
]
, Course : V[SESSION*Course] cRuD -- used for a box
BOX <TABLE>
[ "Course" : I cRud -- used for a field
, "Modules" : isPartOf~ -- CRUD is default
]
, Modules : V[SESSION*Module] cRud
BOX <TABLE>
[ "Modules" : I cRuD
, "Course" : isPartOf cRUd
, "Students" : isEnrolledFor~ CRUD
]
]
The user interface defined by this service is shown as a screenshot below. Notice that the lowercase r in the annotation of the Students box prevents showing the underlying box. The full CRUD functionality in Course yields 'create' functionality (the green plus-button), 'remove pair' functionality (red minus button), and 'delete atom' functionality (the red trash can button). The lowercase c, u, and d in the Modules box prevents displaying that functionality in the user interface.
Column-oriented layout of a user interface with columns in each row
The next sections give some more detailed information on the run time semantics for CRUD annotations as implemented in Ampersand.

Create

CRUD
for a box
for a field.
C
A + (plus) button is displayed that lets you create a new atom, but only if the box-expression is editable.
Enter a new atom and a + button appears. Click the + to add that atom to the listed set of atoms. If you enter an atom that exists (Peter), you can select it.
c
Atoms cannot be created
Atoms cannot be created

Read

Read
CRUD for boxes
CRUD for fields
R
Read is allowed
Read is allowed
r
Read is not allowed
Read is not allowed

Update

Update
CRUD for boxes
CRUD for fields
U
Removing and/or adding a pair (src,tgt) is allowed if expr is editable and the atom exists. Deleting a pair is done with the - button; the atom will NOT be deleted.
Removing and/or adding a pair (src,tgt) is allowed if expr is editable and the atom exists. Deleting a pair is done with the - button; the atom will NOT be deleted.
u
Update is not allowed
Update is not allowed

Delete

Delete
CRUD for boxes
CRUD for fields
D
Deleting a pair is done with the - (minus) button. Deleting an atom is done with the trash bin.
Delete atom (tgt) and all pairs in which it is used.
d
delete not allowed
delete not allowed
A top-level Update or Create are common in my own scripts, e.g. to create an overview of People and be able to create a new Person: INTERFACE "People" : V[SESSION*Person] CRud COLS []. And update is also possible.

Things to remember

  1. 1.
    The red minus is enabled by U. It unlinks an atom (by deleting a pair from a relation) and leaves the atom alone.
  2. 2.
    The red trash bin is enabled by D. It removes an atom and all pairs in which that atom is used.

Background

Motivations for CRUD-functionality are found in the GitHub discussions on CRUD functionality.
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Create
Read
Update
Delete
Things to remember
Background