DOCS: A whole bunch of improvements.

* Changed the ordering so that tutorials come first.
* Added a tutorial on recursive modules.

docsite/site/content/_index.md

@@ -85,23 +85,28 @@ tbody td:nth-child(even) {
 <thead><th></th><th>UCG</th><th>JSonnet</th><th>Dhall</th></thead>
 <tr>
 <td><b>Variables</b></td><td>x</td> <td>x</td><td>x</td>
-<tr>
 </tr>
+<tr>
 <td><b>Conditionals</b></td><td>x</td><td>x</td><td>x</td>
-<tr>
 </tr>
+<tr>
 <td><b>Functions</b></td><td>x</td><td>x</td><td>x</td>
-<tr>
 </tr>
+<tr>
 <td><b>Modules/Classes</b></td><td>x</td><td>x</td><td>x</td>
-<tr>
 </tr>
-<td><b>Imports</b></td><td>x</td><td>x</td><td>x</td>
 <tr>
+<td><b>Imports</b></td><td>x</td><td>x</td><td>x</td>
+</tr>
 <tr>
 <td><b>Std Lib</b></td><td>Minimal</td><td>x</td><td>Prelude</td>
 </tr>
-<tr><td><b>Type Safety</b></td><td>Inferred types and schema validation<td>Unknown</td><td>Inferred types</td></tr>
+<tr>
+<td><b>Type Safety</b></td><td>Inferred types and schema validation<td>Unknown</td><td>Inferred types</td>
+</tr>
+<tr>
+<td><b>Guaranteed to terminate</b></td><td>No</td><td>No</td><td>Yes</td>
+</tr>
 </table>
 <table>
 

docsite/site/content/reference/_index.md

@@ -1,7 +1,7 @@
 +++
 title = "The UCG Language Reference"
 slug = "reference"
-weight = 2
+weight = 3
 sort_by = "weight"
 in_search_index = true
 +++

docsite/site/content/reference/converters.md

@@ -1,5 +1,5 @@
 +++
-title = "UCG Converters"
+title = "Converters"
 weight = 5
 sort_by = "weight"
 in_search_index = true

docsite/site/content/stdlib/_index.md

@@ -1,7 +1,7 @@
 +++
 title = "The UCG Standard Library"
 slug = "stdlib"
-weight = 3
+weight = 4
 sort_by = "weight"
 in_search_index = true
 +++

docsite/site/content/tutorials/_index.md

@@ -1,12 +1,13 @@
 +++
-title = "HowTo Guides"
+title = "Tutorials"
-weight = 4
+weight = 2
 sort_by = "weight"
 in_search_index = true
 +++
-A collection of useful HowTo Guides
+A collection of useful Tutorials
 ----------
 
 Some example use cases for UCG.
 
-* <a href="script">Creating a launch script for a docker container.</a>
+* <a href="script">Creating a launch script for a docker container</a>
+* <a href="recursive-modules">Recursive Modules</a>

docsite/site/content/tutorials/recursive-modules.md

@@ -0,0 +1,159 @@
++++
+title = "Recursive Modules and their Uses"
+slug = "recursive-modules"
+weight = 2
+in_search_index = true
++++
+
+Modules are the only expression in UCG with the capablility to perform
+recursion using self import[^1]. This is safe because statements in a module
+are evaluated at module copy time. This is useful when you need to traverse a
+UCG value deeply. You can see an example of this in the UCG `std/schema.ucg`
+module where we perform a deep type comparison between two UCG values.
+
+As an example we will create a module that can do a dynamic deep index into a
+tuple by using recursion. The module will take in a tuple to traverse and a
+list of field names to descend into.
+
+Create a file called `deep_index.ucg` and put the following into it.
+
+```
+let deep_index = module {
+    tpl = {}, // We expect that tuples here.
+    names = [], // We expect a list of names here.
+} => {
+    // todo
+};
+```
+
+Our `deep_index` module expects a tuple and list of names. We set the defaults
+to an empty tuple and an empty list so anything that isn't either those or NULL
+will be a compile error when we call our module. Next we need to check for the
+existence of the first name in the tuple.
+
+```
+let deep_index = module {
+    tpl = {}, // We expect tuples here.
+    names = [], // We expect a list of names here.
+} => (result) {
+    // Fill in
+
+    let result = select ((mod.names.0) in mod.tpl), NULL, {
+        true = mod.tpl.(mod.names.0),
+    };
+};
+```
+
+There are a few things to note here. We use the out expression `(result)` to
+make this module more ergonomic. We reference our modulr arguments with the
+`mod.` prefix. We also surround part of our selector with parens in
+`(mod.tpl.(mod.names.0))` to force the last selector to pick the field
+dynamically from the result of the grouped expression.
+
+As written this module has a few problems though.
+
+* It is unsafe for inputs where the list of names is empty
+* it is unsafe for inputs where the list of names contains items that are not
+  strings.
+
+## Handling Unsafe inputs.
+
+We want it to be a compile error if anyone calls our module with inputs of the
+wrong shape. We have two possibilities to handle. NULL inputs for either
+argument or an list that contains items that are not strings.
+
+```
+let deep_index = module {
+    tpl = {}, // We expect tuples here.
+    names = [], // We expect a list of names here.
+} => (result) {
+    // import some useful items from the std lib
+    let schema = import "std/schema.ucg";
+    let l = import "std/lists.ucg";
+    
+    // Assert some compile time expectations.
+    // First ensure that names is a list of strings.
+    (names != NULL && schema.shaped{val=names, shape=[""]}) ||
+        fail "names must be non NULL and a list of strings";
+    // Now ensure that tpl is not NULL
+    tpl != NULL || fail "tpl must not be NULL";
+    
+    let select_field = func(name) select (name in mod.tpl), NULL {
+        true = mod.tpl.(name),
+    };
+
+    let result = select l.len(names) > 0, mod.tpl, {
+        true = select_field(mod.names.0),
+    };
+};
+```
+
+Our module will now throw appropriate compile time errors if we pass in invalid
+input shapes. We also correctly handle the case where we have no more names to
+descend into. We now need to handle calling our module recursively.
+
+## Module Recursion
+
+In order to do recursion we have to import ourself.
+
+```
+let deep_index = module {
+    tpl = {}, // We expect that tuples here.
+    names = [], // We expect a list of names here.
+} => (result) {
+    // import some useful items from the std lib
+    let schema = import "std/schema.ucg";
+    let l = import "std/lists.ucg";
+
+    // import ourself now.
+    let self = import "deep_index.ucg".deep_index;
+    
+    // Assert some compile time expectations.
+    // First ensure that names is a list of strings.
+    (names != NULL && schema.shaped{val=names, shape=[""]}) ||
+        fail "names must be non NULL and a list of strings";
+    // Now ensure that tpl is not NULL
+    tpl != NULL || fail "tpl must not be NULL";
+
+    let name = mod.names.0;
+    let fail_msg = "Attempt to descend into a non existent name @" % (name);
+
+    // Field selector function that throws a compile error if the field 
+    // doesn't exist.
+    let select_field = func(name) => select (name in mod.tpl), fail fail_message {
+        true = mod.tpl.(name),
+    };
+
+    // calculate our next value.
+    // If there are no more names then something has gone wrong and
+    // we should hard fail.
+    let next = select l.len(names) > 0, mod.tpl, fail "Impossible" {
+        true = select_field(name),
+    };
+    
+    // we need to get the tail of our list.
+    let tail = l.tail(names);
+
+    // if the tail is non empty then recurse down.
+    // If the tail is empty then return the current value.
+    let result = select l.len(tail) > 0, next, {
+        // recurse
+        true = self{tpl=result, names=tail},
+    };
+};
+```
+
+Our `deep_index` module can now descend down into a tuple given a list of field
+names. If any of the names does not exist it will be a compile failure.
+
+## Final notes
+
+One consquence of this method of doing recursion is that you can not reference
+the module in the file it is defined in. Doing so would result in a compile
+error as UCG detects an import loop. In practice this does not prove to be a
+problem since nearly all usages of the module will be outside of the file it is
+defined in.
+
+<hr>
+
+[^1]: Technnically functions can do recursion if you manually pass in the same function as an argument to itself, but this is unwieldy and error-prone.