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comparison demo/prose @ 777:87a7702d681d

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outer demo
author Adam Chlipala <adamc@hcoop.net>
date Sun, 03 May 2009 14:57:33 -0400
parents 9f2555f06901
children 7b47fc964a0f
comparison
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776:9f2555f06901 777:87a7702d681d
132 <li>The <tt>PRIMARY KEY</tt> constraint establishes the field of the table that we expect to use as a key in looking up specific rows. It is an error for two rows to share the same primary key.</li> 132 <li>The <tt>PRIMARY KEY</tt> constraint establishes the field of the table that we expect to use as a key in looking up specific rows. It is an error for two rows to share the same primary key.</li>
133 <li>The <tt>UNIQUE</tt> constraint is like <tt>PRIMARY KEY</tt>, with the difference being that a table may have many <tt>UNIQUE</tt> constraints but no more than one primary key.</li> 133 <li>The <tt>UNIQUE</tt> constraint is like <tt>PRIMARY KEY</tt>, with the difference being that a table may have many <tt>UNIQUE</tt> constraints but no more than one primary key.</li>
134 <li>The <tt>CHECK</tt> constraint declares a boolean assertion that must hold for every row of the table.</li> 134 <li>The <tt>CHECK</tt> constraint declares a boolean assertion that must hold for every row of the table.</li>
135 <li>The <tt>FOREIGN KEY</tt> constraint declares that a row of the table references a particular column of another table, or of the same table, as we see in this example. It's a static type error to reference a foreign key column that has no <tt>PRIMARY KEY</tt> or <tt>UNIQUE</tt> constraint.</li> 135 <li>The <tt>FOREIGN KEY</tt> constraint declares that a row of the table references a particular column of another table, or of the same table, as we see in this example. It's a static type error to reference a foreign key column that has no <tt>PRIMARY KEY</tt> or <tt>UNIQUE</tt> constraint.</li>
136 </ol> 136 </ol>
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138 outer.urp
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140 <p>SQL outer joins are no problem, as this demo shows. Unlike with SQL, here we have static type inference determining for us which columns may become nullable as a result of an outer join, and the compiler will reject programs that make the wrong assumptions about that process. The details of that nullification don't appear in this example, where the magic of type classes determines both the post-join type of each field and the right pretty-printing and parsing function for each of those types.</p>
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138 sum.urp 142 sum.urp
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140 <p>Metaprogramming is one of the most important facilities of Ur. This example shows how to write a function that is able to sum up the fields of records of integers, no matter which set of fields the particular record has.</p> 144 <p>Metaprogramming is one of the most important facilities of Ur. This example shows how to write a function that is able to sum up the fields of records of integers, no matter which set of fields the particular record has.</p>
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