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See primary documentation in context for Colon pair and colon list

A colon pair is a shorthand syntax used to create or visually present a Pair object. The two most common forms are:

:a(4)          # Same as "a" => 4,   same as Pair.new("a", 4) 
:a<4>          # Same as "a" => "4", same as Pair.new("a", val("4"))

This is also known as the adverbial pair form.

Note: when the part after the colon and before the balanced delimiters is not a legal identifier, other semantics apply, not all of which produce Pair objects.

Two other common forms are:

:a             # Same as :a(True) 
:!a            # Same as :a(False)

A colon list just means that a list that contains only colon pairs, does not need commas, or even spaces:

:a(4):c:!d:c   # Same as a => 4, c => True, d => False, c => True

Finally, if there is a variable with the same name as an intended adverbial pair, you don't have to specify the name twice, but just specify the adverb with the appropriate sigil:

See also Adverb.