Add documentation for select (#747)

docs/SUMMARY.md

@@ -37,6 +37,7 @@
             * [if !](syntax_and_semantics/not.md)
         * [unless](syntax_and_semantics/unless.md)
         * [case](syntax_and_semantics/case.md)
+        * [select](syntax_and_semantics/select.md)
         * [while](syntax_and_semantics/while.md)
             * [break](syntax_and_semantics/break.md)
             * [next](syntax_and_semantics/next.md)

docs/syntax_and_semantics/select.md

@@ -0,0 +1,53 @@
+# select
+
+The `select` expression chooses from a set of blocking operations and proceeds with the branch that becomes available first.
+
+## Syntax
+
+The expression starts with the keyword `select`, followed by a list of one or more `when` branches.
+Each branch has a condition and a body, separated by either
+a statement separator or the keyword `then`.
+Optionally, the last branch may be `else` (without condition).  This denotes the `select` action as non-blocking.
+The expression closes with an `end` keyword.
+
+> NOTE:
+> `select` is similar to a [`case` expression](./case.md) with all branches referring to potentially blocking operations.
+
+Each condition is either a call to a select action or an assignment whose right-hand side is a call to a select action.
+
+```crystal
+select
+when foo = foo_channel.receive
+  puts foo
+when bar = bar_channel.receive?
+  puts bar
+when baz_channel.send
+  exit
+when timeout(5.seconds)
+  puts "Timeout"
+end
+```
+
+## Select actions
+
+A select action call calls a method with the implicit suffix `_select_action`,
+or `_select_action?` for a call with `?` suffix.
+This method returns an instance of the select action.
+
+The `select` expression initiates the select action associated with each branch. If either of them immediately returns, it proceeds with that.
+Otherwise it waits for completion. As soon as one branch completes, all
+others are canceled.
+An `else` branch completes immediately so there will not be any waiting.
+
+Execution continues in the completed branch.
+If the branch condition is an assignment, the result of the select call is assigned to the target variable.
+
+<!-- markdownlint-disable MD046 -->
+
+!!! info "Select actions in the standard library"
+    The standard library provides the following select actions:
+
+    * `Channel#send_select_action`
+    * `Channel#receive_select_action`
+    * `Channel#receive_select_action?`
+    * [`::timeout_select_action`](https://crystal-lang.org/api/toplevel.html#timeout_select_action(timeout:Time::Span):Channel::TimeoutAction-class-method)