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Document top-level do behavior #732

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13 changes: 13 additions & 0 deletions content/reference/special_forms.adoc
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Expand Up @@ -86,6 +86,19 @@ Evaluates _test_. If not the singular values `nil` or `false`, evaluates and yie

Evaluates the expressions __expr__s in order and returns the value of the last. If no expressions are supplied, returns `nil`.

When `do` appears at the top level of a file or at the REPL, each expression within it is evaluated as a separate top-level form. This means each expression is fully evaluated before the next one is compiled, so side effects from earlier expressions (such as `require` or `import`) are visible to the compiler when processing later expressions.

[source,clojure]
----
;; This works because require is evaluated before set/difference is compiled
(do
(require '[clojure.set :as set])
(set/difference #{1 2 3} #{1 3}))
;;=> #{2}
----

This behavior does not apply to `do` forms nested inside other forms, where the entire enclosing form is compiled before any of it is evaluated.

[[let]]
== (`let` [ __binding__* ] __expr__*)

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