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<title>8.14 Extensible variant types</title>
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<h2 class="section" id="s:extensible-variants"><a class="section-anchor" href="#s:extensible-variants" aria-hidden="true"></a>8.14 Extensible variant types</h2>
<ul>
<li><a href="extensiblevariants.html#ss%3Aprivate-extensible">8.14.1 Private extensible variant types</a>
</li></ul>
<p>(Introduced in OCaml 4.02)</p><div class="syntax"><table class="display dcenter"><tr class="c019"><td class="dcell"><table class="c001 cellpading0"><tr><td class="c018">
<a class="syntax" href="typedecl.html#type-representation"><span class="c010">type-representation</span></a></td><td class="c015">::=</td><td class="c017">
...
</td></tr>
<tr><td class="c018"> </td><td class="c015">∣</td><td class="c017"> <span class="c004">=</span> <span class="c004">..</span>
</td></tr>
<tr><td class="c018"> </td></tr>
<tr><td class="c018">
<a class="syntax" href="modtypes.html#specification"><span class="c010">specification</span></a></td><td class="c015">::=</td><td class="c017">
...
</td></tr>
<tr><td class="c018"> </td><td class="c015">∣</td><td class="c017"> <span class="c004">type</span> [<a class="syntax" href="typedecl.html#type-params"><span class="c010">type-params</span></a>] <a class="syntax" href="names.html#typeconstr"><span class="c010">typeconstr</span></a> <a class="syntax" href="#type-extension-spec"><span class="c010">type-extension-spec</span></a>
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<tr><td class="c018"> </td></tr>
<tr><td class="c018">
<a class="syntax" href="modules.html#definition"><span class="c010">definition</span></a></td><td class="c015">::=</td><td class="c017">
...
</td></tr>
<tr><td class="c018"> </td><td class="c015">∣</td><td class="c017"> <span class="c004">type</span> [<a class="syntax" href="typedecl.html#type-params"><span class="c010">type-params</span></a>] <a class="syntax" href="names.html#typeconstr"><span class="c010">typeconstr</span></a> <a class="syntax" href="#type-extension-def"><span class="c010">type-extension-def</span></a>
</td></tr>
<tr><td class="c018"> </td></tr>
<tr><td class="c018">
<a class="syntax" id="type-extension-spec"><span class="c010">type-extension-spec</span></a></td><td class="c015">::=</td><td class="c017"> <span class="c004">+=</span> [<span class="c004">private</span>] [<span class="c004">|</span>] <a class="syntax" href="typedecl.html#constr-decl"><span class="c010">constr-decl</span></a> { <span class="c004">|</span> <a class="syntax" href="typedecl.html#constr-decl"><span class="c010">constr-decl</span></a> }
</td></tr>
<tr><td class="c018"> </td></tr>
<tr><td class="c018">
<a class="syntax" id="type-extension-def"><span class="c010">type-extension-def</span></a></td><td class="c015">::=</td><td class="c017"> <span class="c004">+=</span> [<span class="c004">private</span>] [<span class="c004">|</span>] <a class="syntax" href="#constr-def"><span class="c010">constr-def</span></a> { <span class="c004">|</span> <a class="syntax" href="#constr-def"><span class="c010">constr-def</span></a> }
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<tr><td class="c018"> </td></tr>
<tr><td class="c018">
<a class="syntax" id="constr-def"><span class="c010">constr-def</span></a></td><td class="c015">::=</td><td class="c017">
<a class="syntax" href="typedecl.html#constr-decl"><span class="c010">constr-decl</span></a>
</td></tr>
<tr><td class="c018"> </td><td class="c015">∣</td><td class="c017"> <a class="syntax" href="names.html#constr-name"><span class="c010">constr-name</span></a> <span class="c004">=</span> <a class="syntax" href="names.html#constr"><span class="c010">constr</span></a>
</td></tr>
<tr><td class="c018"> </td></tr>
</table></td></tr>
</table></div><p>Extensible variant types are variant types which can be extended with
new variant constructors. Extensible variant types are defined using
<span class="c003">..</span>. New variant constructors are added using <span class="c003">+=</span>.
</p><div class="caml-example verbatim">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">module</span> Expr = <span class="ocamlkeyword">struct</span>
<span class="ocamlkeyword">type</span> attr = ..
<span class="ocamlkeyword">type</span> attr += Str <span class="ocamlkeyword">of</span> string
<span class="ocamlkeyword">type</span> attr +=
| Int <span class="ocamlkeyword">of</span> int
| Float <span class="ocamlkeyword">of</span> float
<span class="ocamlkeyword">end</span></div></div>
</div><p>Pattern matching on an extensible variant type requires a default case
to handle unknown variant constructors:
</p><div class="caml-example verbatim">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">let</span> to_string = <span class="ocamlkeyword">function</span>
| Expr.Str s -> s
| Expr.Int i -> Int.to_string i
| Expr.Float f -> string_of_float f
| _ -> <span class="ocamlstring">"?"</span></div></div>
</div><p>A preexisting example of an extensible variant type is the built-in
<span class="c003">exn</span> type used for exceptions. Indeed, exception constructors can be
declared using the type extension syntax:
</p><div class="caml-example verbatim">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">type</span> exn += Exc <span class="ocamlkeyword">of</span> int</div></div>
</div><p>Extensible variant constructors can be rebound to a different name. This
allows exporting variants from another module.
</p><div class="caml-example toplevel">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">let</span> not_in_scope = <span class="ocamlhighlight">Str</span> <span class="ocamlstring">"Foo"</span>;;</div>
<div class="pre caml-output error"><span class="ocamlerror">Error</span>: Unbound constructor Str</div></div>
</div><div class="caml-example verbatim">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">type</span> Expr.attr += Str = Expr.Str</div></div>
</div><div class="caml-example toplevel">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">let</span> now_works = Str <span class="ocamlstring">"foo"</span>;;</div>
<div class="pre caml-output ok"><span class="ocamlkeyword">val</span> now_works : Expr.attr = Expr.Str <span class="ocamlstring">"foo"</span></div></div>
</div><p>Extensible variant constructors can be declared <span class="c003">private</span>. As with
regular variants, this prevents them from being constructed directly by
constructor application while still allowing them to be de-structured in
pattern-matching.
</p><div class="caml-example verbatim">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">module</span> B : <span class="ocamlkeyword">sig</span>
<span class="ocamlkeyword">type</span> Expr.attr += <span class="ocamlkeyword">private</span> Bool <span class="ocamlkeyword">of</span> int
<span class="ocamlkeyword">val</span> bool : bool -> Expr.attr
<span class="ocamlkeyword">end</span> = <span class="ocamlkeyword">struct</span>
<span class="ocamlkeyword">type</span> Expr.attr += Bool <span class="ocamlkeyword">of</span> int
<span class="ocamlkeyword">let</span> bool p = <span class="ocamlkeyword">if</span> p <span class="ocamlkeyword">then</span> Bool 1 <span class="ocamlkeyword">else</span> Bool 0
<span class="ocamlkeyword">end</span></div></div>
</div><div class="caml-example toplevel">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">let</span> inspection_works = <span class="ocamlkeyword">function</span>
| B.Bool p -> (p = 1)
| _ -> <span class="ocamlkeyword">true</span>;;</div>
<div class="pre caml-output ok"><span class="ocamlkeyword">val</span> inspection_works : Expr.attr -> bool = <<span class="ocamlkeyword">fun</span>></div></div>
</div><div class="caml-example toplevel">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">let</span> construction_is_forbidden = <span class="ocamlhighlight">B.Bool 1</span>;;</div>
<div class="pre caml-output error"><span class="ocamlerror">Error</span>: Cannot use private constructor Bool to create values of type Expr.attr</div></div>
</div>
<h3 class="subsection" id="ss:private-extensible"><a class="section-anchor" href="#ss:private-extensible" aria-hidden="true"></a>8.14.1 Private extensible variant types</h3>
<p>(Introduced in OCaml 4.06)</p><div class="syntax"><table class="display dcenter"><tr class="c019"><td class="dcell"><table class="c001 cellpading0"><tr><td class="c018">
<a class="syntax" href="typedecl.html#type-representation"><span class="c010">type-representation</span></a></td><td class="c015">::=</td><td class="c017">
...
</td></tr>
<tr><td class="c018"> </td><td class="c015">∣</td><td class="c017"> <span class="c004">=</span> <span class="c004">private</span> <span class="c004">..</span>
</td></tr>
<tr><td class="c018"> </td></tr>
</table></td></tr>
</table></div><p>Extensible variant types can be declared <span class="c003">private</span>. This prevents new
constructors from being declared directly, but allows extension
constructors to be referred to in interfaces.
</p><div class="caml-example verbatim">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">module</span> Msg : <span class="ocamlkeyword">sig</span>
<span class="ocamlkeyword">type</span> t = <span class="ocamlkeyword">private</span> ..
<span class="ocamlkeyword">module</span> MkConstr (X : <span class="ocamlkeyword">sig</span> <span class="ocamlkeyword">type</span> t <span class="ocamlkeyword">end</span>) : <span class="ocamlkeyword">sig</span>
<span class="ocamlkeyword">type</span> t += C <span class="ocamlkeyword">of</span> X.t
<span class="ocamlkeyword">end</span>
<span class="ocamlkeyword">end</span> = <span class="ocamlkeyword">struct</span>
<span class="ocamlkeyword">type</span> t = ..
<span class="ocamlkeyword">module</span> MkConstr (X : <span class="ocamlkeyword">sig</span> <span class="ocamlkeyword">type</span> t <span class="ocamlkeyword">end</span>) = <span class="ocamlkeyword">struct</span>
<span class="ocamlkeyword">type</span> t += C <span class="ocamlkeyword">of</span> X.t
<span class="ocamlkeyword">end</span>
<span class="ocamlkeyword">end</span></div></div>
</div>
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