Improve docs on try/exceptions

public/css/pages.css

@@ -22,6 +22,17 @@
     color: var(--c-text-2);
 }
 
+.markdown h3 {
+    font-size: 1.35rem;
+    line-height: 1.75rem;
+    margin-bottom: 1rem;
+    margin-top: 1.75rem;
+    font-family: Inter, sans-serif;
+    opacity: 0.9;
+    font-weight: 600;
+    color: var(--c-text-2);
+}
+
 .markdown ul {
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     list-style-position: inside;

src/pages/learn/error-handling/try.md

@@ -1,49 +1,150 @@
 ---
 layout: ../../../layouts/PagesLayout.astro
-title: Try expressions
+title: Try/Exceptions
 ---
 
-# Try expressions
+# Try/exceptions
+
+Unlike PHP, in THP all errors must be explicitly declared
+and handled.
+
+## Declare that a function returns an exception
+
+To declare a possible error return value the `Result` enum
+is used.
+
+For example, a function that returned a `DivisionByZero`
+may be written like this:
 
 ```thp
-fun get_value() -> Result[Int, String] { ... }
+fun invert(Int number) -> Result[Int, DivisonByZero]
-
-
-// treating errors as normal enums
-val result = match get_value()
-case Ok(result) { result }
-case Err(error) { return error }
-
-
-// get the value if Ok, otherwise re-throw
-val result = try get_value()
-
-// get the value if Ok, return a new value otherwise
-val result = try get_value() return Err("new error")
-
-// get the value if Ok, assign a new value otherwise
-Int result = try get_value() else 20
-
-// get the value if Ok, otherwise run block with the error value
-val result = try get_value()
-with error
 {
-    // handle error
+    if number == 0
+    {
+        return Err(DivisionByZero())
     }
 
-
+    return Ok(1 / number)
-fun throws_exception() -> Result[Int, MyException|MyOtherException] { ... }
-
-val result = try throws_exception() catch
-case MyException(e)
-{
-    // deal with MyException
-}
-case MyOtherException(e)
-{
-    // deal with MyOtherException
 }
+```
 
+In the previous segment, `Result[Int, DivisionByZero]` denotates
+that the function may return either an `Int` or an `DivisionByZero`.
+
+To return the error we use `Err(...)`, and to return the succes value
+we use `Ok(...)`.
+
+The error may be of any datatype.
+
+
+### Multiple error returns
+
+If there are multiple error types that the function can return,
+you can use the `|` operator:
+
+```thp
+fun sample() -> Result[Int, Exception1|Exception2|Exception3]
+{ /* ... */}
+```
+
+
+
+## Handle an error
+
+Since `Result` is an enum we can use pattern matching to
+get the ok value, or handle the error:
+
+```thp
+val result = match inverse(5)
+case ::Ok(value) { value }
+case ::Err(error) { return error }
+```
+
+However, THP provides syntactic sugar for many common
+patterns for error handling.
+
+## Try expressions
+
+There are several try expressions that simplify error handling:
+
+### Naked try
+
+Using a `try` followed by an expression will execute the
+expression, and if `Ok` is returned, it will return it's value.
+If `Err` is returned, the error will be re-thrown.
+
+```thp
+fun sample(Int x) -> Result[Int, DivisionByZero]
+{
+    val result = try inverse(x)
+
+    result
+}
+```
+
+If `inverse(x)` fails, the error will be returned again.
+
+If `inverse(x)` succeedes, its value will be assigned to `result`.
+
+
+### Try/return
+
+Try/return will run a function and assign its value if `Ok` is found.
+Otherwise, it will return a new value specified by the programmer.
+
+```thp
+fun sample(Int x) -> String
+{
+    val result = try inverse(x) return "0 was found"
+
+    if result == 2 { "2 was found" }
+    else { "other number was found" }
+}
+```
+
+If `inverse(x)` fails, `"0 was found"` will be returned.
+
+If `inverse(x)` succeedes, its value will be assigned to `result`
+and the code will continue to execute normally.
+
+
+### Try/else
+
+Try/else will run an expression and assign its value if `Ok`.
+Otherwise it will assign a second value.
+
+```thp
+fun sample(Int x) -> Int
+{
+    val result = try inverse(x) else 0.0
+
+    result
+}
+```
+
+If `inverse(x)` fails, `0.0` will be assigned to `result`.
+
+If `inverse(x)` succeedes, its value will be assigned to `result`.
+
+
+### Try/catch
+
+Try/catch will run an expression and assign its value if `Ok`.
+Otherwise it will run a block of code, which will handle
+the error and assign a value as well.
+
+```thp
+fun sample(Int x)
+{
+    val result = try inverse(x)
+    catch DivisionByZero e
+    {
+        // Handle `e`
+        // ...
+
+        0.0
+    }
+}
 ```