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doc/manual/source/language/functions.md

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+# Functions
+
+A function is a pure expression that accepts values and returns a value.
+
+## Syntax
+
+```dcdl
+(value: Int) => value + 1
+```
+
+Use ordinary call syntax to invoke a function.
+
+```dcdl
+let
+    increment = (value: Int) => value + 1;
+in
+    increment(41)
+```
+
+A function can have multiple parameters.
+
+```dcdl
+(input_a: { hoge = Int & >= 0; }, input_b: { fuga = Int; }) =>
+{
+    hoge = input_a.hoge;
+    fuga = input_b.fuga;
+}
+```
+
+A parameter range is optional.
+When present, the argument is validated and refined using the same rules as `as` when the argument is referenced.
+
+## Scope and evaluation
+
+Functions have lexical scope and can reference bindings from where they were defined.
+Arguments are evaluated lazily, so an argument not referenced by the function body is not evaluated.
+Recursive field or argument dependencies produce a cycle diagnostic.
+
+Functions can be referenced and called as intermediate values, but cannot be materialized as data.
+An unapplied function remaining in a materialization target produces a diagnostic.
+Function equality is not defined, and composing two function values with `&` produces a conflict.