Create Optimizations.md

Author: antonmedv

docs/Optimizations.md

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+# Optimizations
+
+Expr has a bunch of optimization which will produce more optimal program during compile step.
+
+## In array
+
+```js
+value in ['foo', 'bar', 'baz']
+```
+
+If expr finds an `in` or `not in` expression with an array, it will be transformed into:
+
+```js
+value in {"foo": true, "bar": true, "baz": true}
+```
+
+## Constant folding
+
+Arithmetic expressions with constants is computed on compile step and replaced with result.
+
+```js
+-(2-5)**3-2/(+4-3)+-2
+```
+
+Will be compiled to just single number:
+
+```js
+23
+```
+
+So in expressions it's safe to use some arithmetics for better readability:
+
+```js
+percentage > 0.3 * 100
+```
+
+As it will be simpified to:
+
+```js
+percentage > 30
+```
+
+## In range
+
+```js
+user.Age in 18..32
+```
+
+Will be replaced with binary operator:
+
+```js
+18 <= user.Age && user.Age <= 32
+```
+
+`not in` operator will also work.
+
+## Const range
+
+```js
+1..10_000
+```
+
+Ranges computed on compile stage, repleced with preallocated slices.