unikernel/duniverse/dune_/doc/reference/boolean-language.rst

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+Boolean Language
+================
+
+The Boolean language allows the user to define simple Boolean expressions that
+Dune can evaluate. Here's a semiformal specification of the language:
+
+.. productionlist:: blang
+   op : '=' | '<' | '>' | '<>' | '>=' | '<='
+   expr : (and <expr>+)
+        : (or <expr>+)
+        : (<op> <template> <template>)
+        : (not <expr>)
+        : <template>
+
+After an expression is evaluated, it must be exactly the string ``true`` or
+``false`` to be considered as a Boolean. Any other value will be treated as an
+error.
+
+Below is a simple example of a condition expressing that the build
+has a Flambda compiler, with the help of variable expansion, and is
+targeting OSX:
+
+.. code:: dune
+
+   (and %{ocaml-config:flambda} (= %{ocaml-config:system} macosx))