Implement vars, choose/when, and expressions

Cargo.toml

@@ -5,6 +5,7 @@ members = [
     "examples/esi_example_minimal",
     "examples/esi_example_advanced_error_handling",
     "examples/esi_try_example",
+    "examples/esi_vars_example",
     "examples/esi_example_variants",
 ]
 

README.md

@@ -6,11 +6,19 @@ The implementation is a subset of the [ESI Language Specification 1.0](https://w
 
 - `<esi:include>` (+ `alt`, `onerror="continue"`)
 - `<esi:try>` | `<esi:attempt>` | `<esi:except>`
+- `<esi:vars>` | `<esi:assign>`
+- `<esi:choose>` | `<esi:when>` | `<esi:otherwise>`
 - `<esi:comment>`
 - `<esi:remove>`
 
 Other tags will be ignored and served to the client as-is.
 
+This implementation also includes an expression interpreter and library of functions that can be used. Current functions include:
+
+- `$lower(string)`
+- `$html_encode(string)`
+- `$replace(haystack, needle, replacement [, count])`
+
 ## Example Usage
 
 ```rust,no_run

esi/Cargo.toml

@@ -9,10 +9,12 @@ repository = "https://github.com/fastly/esi"
 readme = "./README.md"
 
 [dependencies]
-quick-xml = "0.36.0"
-thiserror = "^1.0"
+quick-xml = "0.37.1"
+thiserror = "2.0.6"
 fastly = "^0.11"
 log = "^0.4"
+regex = "1.11.1"
+html-escape = "0.2.13"
 
 [dev-dependencies]
 env_logger = "^0.11"

esi/src/error.rs

@@ -41,6 +41,18 @@ pub enum ExecutionError {
     /// Writer error
     #[error("writer error: {0}")]
     WriterError(#[from] std::io::Error),
+
+    /// Expression error
+    #[error("expression failed to evaluate: `{0}`")]
+    ExpressionError(String),
+
+    /// An error occurred while creating a regular expression in an eval context
+    #[error("failed to create a regular expression")]
+    RegexError(#[from] regex::Error),
+
+    /// An error occurred while creating a regular expression in an eval context
+    #[error("failed to execute a function: `{0}`")]
+    FunctionError(String),
 }
 
 pub type Result<T> = std::result::Result<T, ExecutionError>;