Add multiple instantiation.

doc/src/SUMMARY.md

@@ -88,6 +88,7 @@ The Rhai Scripting Language
        4. [Create from AST](language/modules/ast.md)
        5. [Module Resolvers](rust/modules/resolvers.md)
           1. [Custom Implementation](rust/modules/imp-resolver.md)
+   18. [Eval Statement](language/eval.md)
 6. [Safety and Protection](safety/index.md)
    1. [Checked Arithmetic](safety/checked.md)
    2. [Sand-Boxing](safety/sandbox.md)
@@ -119,7 +120,7 @@ The Rhai Scripting Language
       1. [Disable Keywords and/or Operators](engine/disable.md)
       2. [Custom Operators](engine/custom-op.md)
       3. [Extending with Custom Syntax](engine/custom-syntax.md)
-   7. [Eval Statement](language/eval.md)
+   7. [Multiple Instantiation](patterns/multiple.md)
 8. [Appendix](appendix/index.md)
    1. [Keywords](appendix/keywords.md)
    2. [Operators and Symbols](appendix/operators.md)

doc/src/patterns/multiple.md

@@ -0,0 +1,89 @@
+Multiple Instantiation
+======================
+
+{{#include ../links.md}}
+
+
+Background
+----------
+
+Rhai's [features] are not strictly additive.  This is easily deduced from the [`no_std`] feature
+which prepares the crate for `no-std` builds.  Obviously, turning on this feature has a material
+impact on how Rhai behaves.
+
+Many crates resolve this by going the opposite direction: build for `no-std` in default,
+but add a `std` feature, included by default, which builds for the `stdlib`.
+
+
+Rhai Language Features Are Not Additive
+--------------------------------------
+
+Rhai, however, is more complex.  Language features cannot be easily made _additive_.
+
+That is because the _lack_ of a language feature is a feature by itself.
+
+For example, by including [`no_float`], a project sets the Rhai language to ignore floating-point math.
+Floating-point numbers do not even parse under this case and will generate syntax errors.
+Assume that the project expects this behavior (why? perhaps integers are all that make sense
+within the project domain).
+
+Now, assume that a dependent crate also depends on Rhai. Under such circumstances,
+unless _exact_ versioning is used and the dependent crate depends on a _different_ version
+of Rhai, Cargo automatically _merges_ both dependencies, with the [`no_float`] feature turned on
+because Cargo features are _additive_.
+
+This will break the dependent crate, which does not by itself specify [`no_float`]
+and expects floating-point numbers and math to work normally.
+
+There is no way out of this dilemma.  Reversing the [features] set with a `float` feature
+causes the project to break because floating-point numbers are not rejected as expected.
+
+
+Multiple Instantiations of Rhai Within The Same Project
+------------------------------------------------------
+
+The trick is to differentiate between multiple identical copies of Rhai, each having
+a different [features] set, by their _sources_:
+
+* Different versions from [`crates.io`](https://crates.io/crates/rhai/) - The official crate.
+
+* Different releases from [`GitHub`](https://github.com/jonathandturner/rhai) - Crate source on GitHub.
+
+* Forked copy of [https://github.com/jonathandturner/rhai](https://github.com/jonathandturner/rhai) on GitHub.
+
+* Local copy of [https://github.com/jonathandturner/rhai](https://github.com/jonathandturner/rhai) downloaded form GitHub.
+
+Use the following configuration in `Cargo.toml` to pull in multiple copies of Rhai within the same project:
+
+```toml
+[dependencies]
+rhai = { version = "{{version}}", features = [ "no_float" ] }
+rhai_github = { git = "https://github.com/jonathandturner/rhai", features = [ "unchecked" ] }
+rhai_my_github = { git = "https://github.com/my_github/rhai", branch = "variation1", features = [ "serde", "no_closure" ] }
+rhai_local = { path = "../rhai_copy" }
+```
+
+The example above creates four different modules: `rhai`, `rhai_github`, `rhai_my_github` and
+`rhai_local`, each referring to a different Rhai copy with the appropriate [features] set.
+
+Only one crate of any particular version can be used from each source, because Cargo merges
+all candidate cases within the same source, adding all [features] together.
+
+If more than four different instantiations of Rhai is necessary (why?), create more local repositories
+or GitHub forks or branches.
+
+
+Caveat - No Way To Avoid Dependency Conflicts
+--------------------------------------------
+
+Unfortunately, pulling in Rhai from different sources do not resolve the problem of
+[features] conflict between dependencies.  Even overriding `crates.io` via the `[patch]` manifest
+section doesn't work - all dependencies will eventually find the only one copy.
+
+What is necessary - multiple copies of Rhai, one for each dependent crate that requires it,
+together with their _unique_ [features] set intact.  In other words, turning off Cargo's
+crate merging feature _just for Rhai_.
+
+Unfortunately, as of this writing, there is no known method to achieve it.
+
+Therefore, moral of the story: avoid pulling in multiple crates that depend on Rhai.

doc/src/start/features.md

@@ -52,3 +52,15 @@ no floating-point, is `Send + Sync` (so it can be safely used across threads), a
 nor loading external [modules].
 
 This configuration is perfect for an expression parser in a 32-bit embedded system without floating-point hardware.
+
+
+Caveat - Features Are Not Additive
+---------------------------------
+
+Rhai features are not strictly _additive_ - i.e. they do not only add optional functionalities.
+
+In fact, most features are _subtractive_ - i.e. they _remove_ functionalities.
+
+There is a reason for this design, because the _lack_ of a language feature by itself is a feature.
+
+See [here]({{rootUrl}}/patterns/multiple.md) for more details.