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better explanation of the purpose of type unions

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Co-authored-by: Pavel Avgustinov <54942558+p0@users.noreply.github.com>
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Philip Ginsbach
2020-05-29 10:22:57 +01:00
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parent 0562b4a2dd
commit 299d87aa8e
1 changed files with 3 additions and 2 deletions
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docs/language/ql-handbook/types.rst
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@@ -497,8 +497,9 @@ Type unions are used for creating restricted subsets of an existing :ref:`algebr
selecting a subset of the branches of that datatype and binding them to a new type.
Type unions of :ref:`database types <database-types>` are also supported.
You can use a type union to explicitly restrict the permitted branches from an algebraic datatype
and resolve spurious :ref:`recursion <recursion>` in predicates.
You can use a type union to give a name to a subset of the branches from an algebraic datatype.
In some cases, using the type union over the whole algebraic datatype can avoid spurious
:ref:`recursion <recursion>` in predicates.
For example, the following construction is legal::
newtype InitialValueSource =