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Scope modifiers: Difference between revisions
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done</pre>
As you can see, these are very powerful capabilities which make it trivial to write control structures in next to no Tcl code at all.
=={{header|Ursala}}==
There are no variables in Ursala except dummy variables used in
lambda abstractions, but scope rules govern the visibility of
constants and function declarations.
When compiling a library, directives such as #library and #binary can
be switched on and off throughout a source text, and only the symbols
declared when they're on will become visible library entry points.
<lang Ursala>
local_shop = 0
hidden_variable = 3
#library+
this_public_constant = local_shop
a_visible_function = +
#library-
for_local_people = 7
</lang>
By default, every symbol is visible to every other within the same
file, but the scope modifiers #hide and #export can create multiple
scopes within a single file. In this example, the symbol x will have
a value of 1,
<lang Ursala>
foo = 1
#hide+
foo = 2
bar = 3
#hide-
x = foo
</lang>
but it will be 2 in this example, where
the #export directive selectively allows an otherwise
hidden declaration to be visible outside its enclosing
scope.
<lang Ursala>
foo = 1
#hide+
#export+
foo = 2
#export-
bar = 3
#hide-
x = foo
</lang>
The #hide directives can be arbitrarily nested in matched pairs
to create block structured scope, but doing so is likely to be
overkill.
When name clashes occur between imported and locally declared
symbols, they are resolved by default in favor of the local
declaration. However, this behavior can be overridden using
the dash operator as shown.
<lang Ursala>
#import std
cat = 3
a_string = std-cat('foo','bar')
</lang>
Here, std-cat refers to the concatenation function from the standard
library, not the locally declared constant by that name.
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