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<title>8.17 Inline records</title>
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<h2 class="section" id="s:inline-records"><a class="section-anchor" href="#s:inline-records" aria-hidden="true"></a>8.17 Inline records</h2>
<p>
(Introduced in OCaml 4.03)
</p><div class="syntax"><table class="display dcenter"><tr class="c019"><td class="dcell"><table class="c001 cellpading0"><tr><td class="c018">
<span class="c010">constr-args</span></td><td class="c015">::=</td><td class="c017">
...
</td></tr>
<tr><td class="c018"> </td><td class="c015">∣</td><td class="c017"> <a class="syntax" href="typedecl.html#record-decl"><span class="c010">record-decl</span></a>
</td></tr>
<tr><td class="c018"> </td></tr>
</table></td></tr>
</table></div><p>The arguments of sum-type constructors can now be defined using the
same syntax as records. Mutable and polymorphic fields are allowed.
GADT syntax is supported. Attributes can be specified on individual
fields.</p><p>Syntactically, building or matching constructors with such an inline
record argument is similar to working with a unary constructor whose
unique argument is a declared record type. A pattern can bind
the inline record as a pseudo-value, but the record cannot escape the
scope of the binding and can only be used with the dot-notation to
extract or modify fields or to build new constructor values.</p><div class="caml-example verbatim">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">type</span> t =
| Point <span class="ocamlkeyword">of</span> {width: int; <span class="ocamlkeyword">mutable</span> x: float; <span class="ocamlkeyword">mutable</span> y: float}
| Other
<span class="ocamlkeyword">let</span> v = Point {width = 10; x = 0.; y = 0.}
<span class="ocamlkeyword">let</span> scale l = <span class="ocamlkeyword">function</span>
| Point p -> Point {p <span class="ocamlkeyword">with</span> x = l *. p.x; y = l *. p.y}
| Other -> Other
<span class="ocamlkeyword">let</span> print = <span class="ocamlkeyword">function</span>
| Point {x; y; _} -> Printf.printf <span class="ocamlstring">"%f/%f"</span> x y
| Other -> ()
<span class="ocamlkeyword">let</span> reset = <span class="ocamlkeyword">function</span>
| Point p -> p.x <- 0.; p.y <- 0.
| Other -> ()</div></div>
</div><div class="caml-example verbatim">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">let</span> invalid = <span class="ocamlkeyword">function</span>
| Point p -> <span class="ocamlhighlight">p</span></div>
<div class="pre caml-output error"><span class="ocamlerror">Error</span>: This form is not allowed as the type of the inlined record could escape.</div></div>
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