There are various ways to provide such support: MetaEdit+ uses regular
expressions to specify legal values; IPSYS TBK had a text layout language that could
handle simple cases, for example, free-form text separated by commas.
Constraints on property values can also look beyond the string itself to the wider
context of the object and even graph where it is being entered. For instance, there may
be a constraint that no two objects of a certain type could have the same name, either
globally or at least not in the same graph. While such constraints could be handwritten
in code, they occur frequently enough that supporting them in the metamodeling
language is useful.
Explicit Concept of Role Few?¬?rst versions of tools pay suf?¬?cient attention to the
connections between objects. Even a casual examination of areas where graphical
languages provide bene?¬?ts over their textual counterparts reveals that visual links
between objects are a major advantage. Anybody who has tried to read an XML ?¬?le
where elements need to refer to each other across the tree structure will be aware of
how dif?¬?cult it is to see such information: a graphical representation shows this
information instantly. Since the graphical representation must also show the tree
structure, there will be more than one kind of link between objects.
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