diff --git a/hornbeam_grammar/src/ast.rs b/hornbeam_grammar/src/ast.rs
index e1f44a3..495f177 100644
--- a/hornbeam_grammar/src/ast.rs
+++ b/hornbeam_grammar/src/ast.rs
@@ -4,9 +4,17 @@ use std::collections::BTreeMap;
 
 #[derive(Clone, Debug, Eq, PartialEq, Serialize)]
 pub struct Template {
+    pub param_defs: Option<Vec<ParameterDefinition>>,
     pub blocks: Vec<Block>,
 }
 
+#[derive(Clone, Debug, Eq, PartialEq, Serialize)]
+pub struct ParameterDefinition {
+    pub name: IStr,
+    pub loc: Locator,
+    pub default: Option<Expression>,
+}
+
 #[derive(Clone, Debug, Eq, PartialEq, Serialize)]
 pub enum Block {
     HtmlElement(HtmlElement),
diff --git a/hornbeam_grammar/src/hornbeam.pest b/hornbeam_grammar/src/hornbeam.pest
index 4d03351..e0b8a7e 100644
--- a/hornbeam_grammar/src/hornbeam.pest
+++ b/hornbeam_grammar/src/hornbeam.pest
@@ -1,5 +1,21 @@
 //
-Hornbeam = { SOI ~ wsnl* ~ BlockContent* ~ ws* ~ EOI }
+Hornbeam = { SOI ~ wsnl* ~ PreambleList? ~ wsnl* ~ HornbeamBlockList ~ ws* ~ EOI }
+
+// Will eventually expand to other types of preamble content
+PreambleList = {
+    // accept `declare` keyword. We expect `PEEK_ALL` to be empty, but future definitions might change this.
+    PEEK_ALL ~ "declare" ~ lineEnd ~
+    // then accept the first definition. We must have at least one definition.
+    // We accept the new indentation level with the `PUSH` here
+    PEEK_ALL ~ PUSH("  "+ | "\t"+) ~ ParameterDefinition ~
+    // Now accept any number of extra definitions at the same indentation level.
+    (PEEK_ALL ~ ParameterDefinition)* ~
+    // Drop the indentation when exiting the preamble block
+    DROP
+}
+
+
+HornbeamBlockList = { BlockContent* }
 
 NewBlock = _{
 	PEEK_ALL ~ PUSH("  "+ | "\t"+) ~ BlockContent ~
@@ -18,6 +34,13 @@ BlockContent = _{
 	DefineFragment
 }
 
+// `param $x`
+// `param $x = 1 + 1`
+// TODO: type annotation: `param $x: int`
+ParameterDefinition = {
+    "param" ~ ws+ ~ "$" ~ Identifier ~ (ws* ~ "=" ~ ws* ~ Expr)? ~ lineEnd
+}
+
 Element = {
 	ElementName ~ cssClass* ~ domId? ~ (ws+ ~ AttrMapLiteral)? ~ lineEnd ~ (NewBlock | NewSlotBlock)?
 }
diff --git a/hornbeam_grammar/src/parser.rs b/hornbeam_grammar/src/parser.rs
index f7f4fb5..4422db3 100644
--- a/hornbeam_grammar/src/parser.rs
+++ b/hornbeam_grammar/src/parser.rs
@@ -2,8 +2,8 @@
 
 use crate::ast::{
     Binding, Block, ComponentElement, DefineExpandSlot, DefineFragment, ElementAttributeFlags,
-    Expression, ForBlock, HtmlElement, IfBlock, MatchBinding, MatchBlock, SetStatement, StringExpr,
-    StringPiece, Template,
+    Expression, ForBlock, HtmlElement, IfBlock, MatchBinding, MatchBlock, ParameterDefinition,
+    SetStatement, StringExpr, StringPiece, Template,
 };
 use crate::{intern, IStr, Locator};
 use lazy_static::lazy_static;
@@ -61,8 +61,51 @@ lazy_static! {
 #[pest_consume::parser]
 impl HornbeamParser {
     fn Hornbeam(input: Node) -> PCResult<Template> {
-        let blocks = HornbeamParser::helper_blocks(input.into_children())?;
-        Ok(Template { blocks })
+        Ok(match_nodes!(input.into_children();
+            [PreambleList(param_defs), HornbeamBlockList(blocks), EOI(_)] => {
+                Template { param_defs, blocks }
+            },
+            [HornbeamBlockList(blocks), EOI(_)] => {
+                Template { param_defs: None, blocks }
+            }
+        ))
+    }
+
+    fn PreambleList(input: Node) -> PCResult<Option<Vec<ParameterDefinition>>> {
+        let param_defs: Vec<ParameterDefinition> = match_nodes!(input.into_children();
+            [ParameterDefinition(param_defs)..] => param_defs.collect()
+        );
+        // Templates with no parameter definitions are treated as untyped.
+        // (Mostly for backwards compat)
+        Ok(if param_defs.is_empty() {
+            None
+        } else {
+            Some(param_defs)
+        })
+    }
+
+    fn ParameterDefinition(input: Node) -> PCResult<ParameterDefinition> {
+        let loc = nodeloc(&input);
+        Ok(match_nodes!(input.into_children();
+            [Identifier(name), Expr(default)] => {
+                ParameterDefinition {
+                    name,
+                    loc,
+                    default: Some(default),
+                }
+            },
+            [Identifier(name)] => {
+                ParameterDefinition {
+                    name,
+                    loc,
+                    default: None,
+                }
+            },
+        ))
+    }
+
+    fn HornbeamBlockList(input: Node) -> PCResult<Vec<Block>> {
+        HornbeamParser::helper_blocks(input.into_children())
     }
 
     fn Element(input: Node) -> PCResult<Block> {
@@ -617,6 +660,23 @@ div
         .unwrap());
     }
 
+    #[test]
+    fn param_defs() {
+        assert_yaml_snapshot!(parse_template(
+            r#"
+declare
+    // We need form information
+    param $form
+
+    // Also wouldn't hurt to have a default value on this other parameter
+
+    param $user = None
+        "#,
+            "inp"
+        )
+        .unwrap());
+    }
+
     #[test]
     fn supply_slots_to_components_only() {
         assert_debug_snapshot!(parse_template(
diff --git a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__for_blocks.snap b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__for_blocks.snap
index 8112862..6c2c018 100644
--- a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__for_blocks.snap
+++ b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__for_blocks.snap
@@ -2,6 +2,7 @@
 source: hornbeam_grammar/src/parser.rs
 expression: "parse_template(r#\"\nfor $x in $xs\n    for $y in $ys\n        \"Woot\"\n    empty\n        \"no ys\"\nempty\n    \"no xs\"\n    \"#,\n        \"inp\").unwrap()"
 ---
+param_defs: ~
 blocks:
   - ForBlock:
       binding:
@@ -44,4 +45,3 @@ blocks:
         filename: inp
         line: 2
         column: 1
-
diff --git a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__fragments_and_slots.snap b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__fragments_and_slots.snap
index 9cc7605..680eeca 100644
--- a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__fragments_and_slots.snap
+++ b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__fragments_and_slots.snap
@@ -2,6 +2,7 @@
 source: hornbeam_grammar/src/parser.rs
 expression: "parse_template(r#\"\ndiv\n    fragment BobbyDazzler\n        span\n            \"Wow!\"\n    fragment MainPart\n        slot :main\n        div\n            optional slot :footer\n        \"#,\n        \"inp\").unwrap()"
 ---
+param_defs: ~
 blocks:
   - HtmlElement:
       name: div
@@ -64,4 +65,3 @@ blocks:
         filename: inp
         line: 2
         column: 1
-
diff --git a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__if_blocks-2.snap b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__if_blocks-2.snap
index c15ad5b..4dd90ad 100644
--- a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__if_blocks-2.snap
+++ b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__if_blocks-2.snap
@@ -2,6 +2,7 @@
 source: hornbeam_grammar/src/parser.rs
 expression: "parse_template(r#\"\nif 1 + 1 == 2\n  div\n    \"Phew, safe!\"\nelse if 1 + 2 - 1 == 3 // not too far off, I suppose\n  Warning\n    \"Not quite, but fairly close. What kind of world is this?\"\nelse // peculiar.\n  \"Not even close, eh?\"\n        \"#,\n        \"inp\").unwrap()"
 ---
+param_defs: ~
 blocks:
   - IfBlock:
       condition:
@@ -76,4 +77,3 @@ blocks:
         filename: inp
         line: 2
         column: 1
-
diff --git a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__if_blocks.snap b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__if_blocks.snap
index 4817ad0..98ec781 100644
--- a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__if_blocks.snap
+++ b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__if_blocks.snap
@@ -2,6 +2,7 @@
 source: hornbeam_grammar/src/parser.rs
 expression: "parse_template(r#\"\nif 10 / 2 == 5 or $point.x == 42 // div for a div?\n    div\n        \"#,\n        \"inp\").unwrap()"
 ---
+param_defs: ~
 blocks:
   - IfBlock:
       condition:
@@ -54,4 +55,3 @@ blocks:
         filename: inp
         line: 2
         column: 1
-
diff --git a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__localisation.snap b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__localisation.snap
index ec07c13..3cb583a 100644
--- a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__localisation.snap
+++ b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__localisation.snap
@@ -2,6 +2,7 @@
 source: hornbeam_grammar/src/parser.rs
 expression: "parse_template(r#\"\ndiv\n    span\n        @header-wow\n    MainBody\n        ''\n        @body-welcome\n        @body-msg{count = $messages.len()}\n        ''\n    Footer\n        @footer-copyright{year = today().year}\n        \"#,\n        \"inp\").unwrap()"
 ---
+param_defs: ~
 blocks:
   - HtmlElement:
       name: div
@@ -90,4 +91,3 @@ blocks:
         filename: inp
         line: 2
         column: 1
-
diff --git a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__match_blocks.snap b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__match_blocks.snap
index 8f92b09..be4f829 100644
--- a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__match_blocks.snap
+++ b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__match_blocks.snap
@@ -2,6 +2,7 @@
 source: hornbeam_grammar/src/parser.rs
 expression: "parse_template(r#\"\nif $x\n    match $y\n        None =>\n            \"None\"\n        Some($z) =>\n            \"Some(${$z})\"\n    \"#,\n        \"inp\").unwrap()"
 ---
+param_defs: ~
 blocks:
   - IfBlock:
       condition:
@@ -50,4 +51,3 @@ blocks:
         filename: inp
         line: 2
         column: 1
-
diff --git a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__param_defs.snap b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__param_defs.snap
new file mode 100644
index 0000000..cb85d87
--- /dev/null
+++ b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__param_defs.snap
@@ -0,0 +1,18 @@
+---
+source: hornbeam_grammar/src/parser.rs
+expression: "parse_template(r#\"\ndeclare\n    // We need form information\n    param $form\n\n    // Also wouldn't hurt to have a default value on this other parameter\n\n    param $user = None\n        \"#,\n        \"inp\").unwrap()"
+---
+param_defs:
+  - name: form
+    loc:
+      filename: inp
+      line: 4
+      column: 5
+    default: ~
+  - name: user
+    loc:
+      filename: inp
+      line: 8
+      column: 5
+    default: NoneLiteral
+blocks: []
diff --git a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__raw.snap b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__raw.snap
index 48fe2b0..ec71239 100644
--- a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__raw.snap
+++ b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__raw.snap
@@ -2,6 +2,7 @@
 source: hornbeam_grammar/src/parser.rs
 expression: "parse_template(r#\"\ndiv\n    span\n        raw \"<u>wow $x ${$x} @wowage{}</u>\"\n            \"#,\n        \"inp\").unwrap()"
 ---
+param_defs: ~
 blocks:
   - HtmlElement:
       name: div
@@ -42,4 +43,3 @@ blocks:
         filename: inp
         line: 2
         column: 1
-
diff --git a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__simple_parses_correct.snap b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__simple_parses_correct.snap
index 7b03b4a..1168965 100644
--- a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__simple_parses_correct.snap
+++ b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__simple_parses_correct.snap
@@ -2,6 +2,7 @@
 source: hornbeam_grammar/src/parser.rs
 expression: "parse_template(r#\"\n// This is a simple Hornbeam template that just shows a <div>\ndiv\n        \"#,\n        \"inp\").unwrap()"
 ---
+param_defs: ~
 blocks:
   - HtmlElement:
       name: div
@@ -13,4 +14,3 @@ blocks:
         filename: inp
         line: 3
         column: 1
-
diff --git a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__string_interpolations_nested.snap b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__string_interpolations_nested.snap
index 514f802..f4bd385 100644
--- a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__string_interpolations_nested.snap
+++ b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__string_interpolations_nested.snap
@@ -2,6 +2,7 @@
 source: hornbeam_grammar/src/parser.rs
 expression: "parse_template(r#\"\nMyComponent\n    :someslot\n        ''\n        ${\"abc\" + \"def${ 1 + 1 }\"}\n        Not too bad now.\n        ''\n        \"#,\n        \"inp\").unwrap()"
 ---
+param_defs: ~
 blocks:
   - ComponentElement:
       name: MyComponent
@@ -34,4 +35,3 @@ blocks:
         filename: inp
         line: 2
         column: 1
-
diff --git a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__supply_slots_to_components_only-2.snap b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__supply_slots_to_components_only-2.snap
index 5d620cb..69e2777 100644
--- a/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__supply_slots_to_components_only-2.snap
+++ b/hornbeam_grammar/src/snapshots/hornbeam_grammar__parser__tests__supply_slots_to_components_only-2.snap
@@ -2,6 +2,7 @@
 source: hornbeam_grammar/src/parser.rs
 expression: "parse_template(r#\"\nMyComponent\n    :someslot\n        \"That's better!\"\n        \"#,\n        \"inp\").unwrap()"
 ---
+param_defs: ~
 blocks:
   - ComponentElement:
       name: MyComponent
@@ -15,4 +16,3 @@ blocks:
         filename: inp
         line: 2
         column: 1
-
diff --git a/hornbeam_interpreter/src/engine.rs b/hornbeam_interpreter/src/engine.rs
index 9764125..0586bc3 100644
--- a/hornbeam_interpreter/src/engine.rs
+++ b/hornbeam_interpreter/src/engine.rs
@@ -475,12 +475,61 @@ impl<'a, O: OutputSystem + Send, LS: LocalisationSystem + Sync + Send> Interpret
                 // TODO check slots
                 // ...
 
-                // check params
+                // check params and evaluate defaults
+                if let Some(param_defs) = &module.param_defs {
+                    for param_def in param_defs {
+                        if evaled_args.contains_key(param_def.name.as_str()) {
+                            continue;
+                        }
 
+                        let Some(default_expr) = &param_def.default else {
+                            return Err(InterpreterError::TypeError {
+                                context: "Call".to_string(),
+                                conflict: format!(
+                                    "missing required parameter {} for {name:?}.",
+                                    param_def.name
+                                ),
+                                location: step.locator.clone(),
+                            });
+                        };
+
+                        // Push a temporary empty scope
+                        // TODO in the future allow evaluation with some scope if desired.
+                        self.scopes.push(Scope {
+                            variables: BTreeMap::new(),
+                            slots: BTreeMap::new(),
+                        });
+                        evaled_args.insert(
+                            String::from(param_def.name.as_str()),
+                            self.evaluate_expression(
+                                scope_idx + 1,
+                                default_expr,
+                                &param_def.locator,
+                            )?,
+                        );
+                        self.scopes.pop();
+                    }
+                    for supplied_param in evaled_args.keys() {
+                        if !param_defs
+                            .iter()
+                            .any(|def| def.name.as_str() == supplied_param)
+                        {
+                            return Err(InterpreterError::TypeError {
+                                context: "Call".to_string(),
+                                conflict: format!(
+                                    "provided non-existent parameter {} for {name:?}.",
+                                    supplied_param
+                                ),
+                                location: Locator::empty(),
+                            });
+                        }
+                    }
+                }
                 self.scopes.push(Scope {
                     variables: evaled_args,
                     slots: filled_in_slots,
                 });
+
                 let next_scope_idx = self.scopes.len() - 1;
 
                 self.run_steps(next_scope_idx, &module.steps).await?;
@@ -902,7 +951,55 @@ impl<'a, O: OutputSystem + Send, LS: LocalisationSystem + Sync + Send> Interpret
                 location: Locator::empty(),
             });
         };
-        // TODO slot + params check
+        // TODO deduplicate with `Call` step
+        if let Some(param_defs) = &main.param_defs {
+            for param_def in param_defs {
+                if self.scopes[0]
+                    .variables
+                    .contains_key(param_def.name.as_str())
+                {
+                    continue;
+                }
+
+                let Some(default_expr) = &param_def.default else {
+                    return Err(InterpreterError::TypeError {
+                        context: "main entrypoint".to_string(),
+                        conflict: format!(
+                            "missing required parameter {} for entrypoint {:?}.",
+                            param_def.name, self.entrypoint
+                        ),
+                        location: Locator::empty(),
+                    });
+                };
+
+                // Push a temporary empty scope
+                // TODO in the future allow evaluation with some scope if desired.
+                self.scopes.push(Scope {
+                    variables: BTreeMap::new(),
+                    slots: BTreeMap::new(),
+                });
+                let value = self.evaluate_expression(1, default_expr, &param_def.locator)?;
+                self.scopes[0]
+                    .variables
+                    .insert(String::from(param_def.name.as_str()), value);
+                self.scopes.pop();
+            }
+            for supplied_param in self.scopes[0].variables.keys() {
+                if !param_defs
+                    .iter()
+                    .any(|def| def.name.as_str() == supplied_param)
+                {
+                    return Err(InterpreterError::TypeError {
+                        context: "main entrypoint".to_string(),
+                        conflict: format!(
+                            "provided non-existent parameter {} for entrypoint {:?}.",
+                            supplied_param, self.entrypoint
+                        ),
+                        location: Locator::empty(),
+                    });
+                }
+            }
+        }
         self.run_steps(0, &main.steps).await?;
         Ok(())
     }
diff --git a/hornbeam_interpreter/tests/snapshots.rs b/hornbeam_interpreter/tests/snapshots.rs
index 1deb5bc..2fff0e9 100644
--- a/hornbeam_interpreter/tests/snapshots.rs
+++ b/hornbeam_interpreter/tests/snapshots.rs
@@ -19,6 +19,7 @@ fn simple_test_struct() -> SimpleTestStruct {
     }
 }
 
+#[track_caller]
 fn simple_render(template: &str) -> String {
     let mut templates = LoadedTemplates::new(DebugLocalisationSystem);
     templates
@@ -112,3 +113,18 @@ match $unused_var
         "#
     ))
 }
+
+#[test]
+fn snapshot_006() {
+    assert_snapshot!(simple_render(
+        r#"
+declare
+    param $default_param = "default value!"
+
+    param $five
+    param $sts
+
+"$default_param"
+        "#
+    ))
+}
diff --git a/hornbeam_interpreter/tests/snapshots/snapshots__snapshot_006.snap b/hornbeam_interpreter/tests/snapshots/snapshots__snapshot_006.snap
new file mode 100644
index 0000000..ddc747b
--- /dev/null
+++ b/hornbeam_interpreter/tests/snapshots/snapshots__snapshot_006.snap
@@ -0,0 +1,5 @@
+---
+source: hornbeam_interpreter/tests/snapshots.rs
+expression: "simple_render(r#\"\ndeclare\n    param $default_param = \"default value!\"\n\n\"$default_param\"\n        \"#)"
+---
+default value!
diff --git a/hornbeam_ir/src/ast_to_ir.rs b/hornbeam_ir/src/ast_to_ir.rs
index fbd1e50..fe97778 100644
--- a/hornbeam_ir/src/ast_to_ir.rs
+++ b/hornbeam_ir/src/ast_to_ir.rs
@@ -1,4 +1,4 @@
-use crate::ir::{Step, StepDef, TemplateFunction};
+use crate::ir::{ParamDef, Step, StepDef, TemplateFunction};
 use hornbeam_grammar::ast::{
     Binding, Block, Expression, HtmlElement, MatchBinding, StringExpr, StringPiece, Template,
 };
@@ -36,7 +36,7 @@ pub enum AstToIrError {
 pub(crate) fn pull_out_entrypoints(
     mut template: Template,
     template_name: &str,
-) -> Result<BTreeMap<String, Vec<Block>>, AstToIrError> {
+) -> Result<BTreeMap<String, Template>, AstToIrError> {
     let mut functions = BTreeMap::new();
 
     for child in &mut template.blocks {
@@ -45,7 +45,7 @@ pub(crate) fn pull_out_entrypoints(
 
     match functions.entry(template_name.to_owned()) {
         Entry::Vacant(ve) => {
-            ve.insert(template.blocks);
+            ve.insert(template);
         }
         Entry::Occupied(_) => {
             return Err(AstToIrError::SemanticError {
@@ -66,7 +66,7 @@ pub(crate) fn pull_out_entrypoints(
 fn pull_out_entrypoints_from_block(
     block: &mut Block,
     template_name: &str,
-    target: &mut BTreeMap<String, Vec<Block>>,
+    target: &mut BTreeMap<String, Template>,
 ) -> Result<(), AstToIrError> {
     match block {
         Block::HtmlElement(he) => {
@@ -122,7 +122,10 @@ fn pull_out_entrypoints_from_block(
                     // function.
                     let mut blocks = Vec::new();
                     std::mem::swap(&mut blocks, &mut frag.blocks);
-                    ve.insert(blocks);
+                    ve.insert(Template {
+                        blocks,
+                        param_defs: None,
+                    });
                 }
                 Entry::Occupied(_) => {
                     return Err(AstToIrError::SemanticError {
@@ -143,22 +146,25 @@ fn pull_out_entrypoints_from_block(
 
 /// Step 2. Compile the AST to IR steps.
 pub(crate) fn compile_functions(
-    functions: &BTreeMap<String, Vec<Block>>,
+    functions: &BTreeMap<String, Template>,
 ) -> Result<BTreeMap<String, TemplateFunction>, AstToIrError> {
     let mut result = BTreeMap::new();
-    for (func_name, func_blocks) in functions {
+    for (func_name, func_template) in functions {
         let mut steps = Vec::new();
-        for block in func_blocks {
+        for block in &func_template.blocks {
             compile_ast_block_to_steps(block, &mut steps)?;
         }
-        // TODO save more information
-        result.insert(
-            func_name.clone(),
-            TemplateFunction {
-                param_defs: None,
-                steps,
-            },
-        );
+
+        let param_defs = func_template.param_defs.as_ref().map(|defs| {
+            defs.iter()
+                .map(|def| ParamDef {
+                    name: def.name.clone(),
+                    default: def.default.clone(),
+                    locator: def.loc.clone(),
+                })
+                .collect()
+        });
+        result.insert(func_name.clone(), TemplateFunction { param_defs, steps });
     }
     Ok(result)
 }
@@ -642,4 +648,24 @@ div.stylish#myid {size=42, stringy="yup", arb=$ritrary}
         )
         .unwrap());
     }
+    #[test]
+    fn test_compile_params() {
+        let template = parse_template(
+            r#"
+declare
+    param $ritary
+
+    param $three = 3
+
+div {arb=$ritrary}
+    OtherComponent {param3=$three}
+        "#,
+            "inp",
+        )
+        .unwrap();
+        assert_yaml_snapshot!(compile_functions(
+            &pull_out_entrypoints(template, "TemplateName").unwrap()
+        )
+        .unwrap());
+    }
 }
diff --git a/hornbeam_ir/src/ir.rs b/hornbeam_ir/src/ir.rs
index 2312ae8..be0936e 100644
--- a/hornbeam_ir/src/ir.rs
+++ b/hornbeam_ir/src/ir.rs
@@ -23,6 +23,7 @@ pub struct ParamDef {
     pub name: IStr,
     // TODO type information
     pub default: Option<Expression>,
+    pub locator: Locator,
 }
 
 #[derive(Clone, Debug, Eq, PartialEq, Serialize)]
diff --git a/hornbeam_ir/src/snapshots/hornbeam_ir__ast_to_ir__tests__compile_params.snap b/hornbeam_ir/src/snapshots/hornbeam_ir__ast_to_ir__tests__compile_params.snap
new file mode 100644
index 0000000..89b64c3
--- /dev/null
+++ b/hornbeam_ir/src/snapshots/hornbeam_ir__ast_to_ir__tests__compile_params.snap
@@ -0,0 +1,85 @@
+---
+source: hornbeam_ir/src/ast_to_ir.rs
+expression: "compile_functions(&pull_out_entrypoints(template,\n                    \"TemplateName\").unwrap()).unwrap()"
+---
+TemplateName:
+  param_defs:
+    - name: ritary
+      default: ~
+      locator:
+        filename: inp
+        line: 3
+        column: 5
+    - name: three
+      default:
+        IntLiteral:
+          val: 3
+      locator:
+        filename: inp
+        line: 5
+        column: 5
+  steps:
+    - WriteLiteral:
+        escape: false
+        text: "<div"
+      locator:
+        filename: inp
+        line: 7
+        column: 1
+    - WriteLiteral:
+        escape: false
+        text: " arb=\""
+      locator:
+        filename: inp
+        line: 7
+        column: 1
+    - WriteEval:
+        escape: true
+        expr:
+          Variable:
+            name: ritrary
+            loc:
+              filename: inp
+              line: 7
+              column: 10
+      locator:
+        filename: inp
+        line: 7
+        column: 1
+    - WriteLiteral:
+        escape: false
+        text: "\""
+      locator:
+        filename: inp
+        line: 7
+        column: 1
+    - WriteLiteral:
+        escape: false
+        text: ">"
+      locator:
+        filename: inp
+        line: 7
+        column: 1
+    - Call:
+        name: OtherComponent
+        args:
+          param3:
+            Variable:
+              name: three
+              loc:
+                filename: inp
+                line: 8
+                column: 28
+        slots:
+          main: []
+      locator:
+        filename: inp
+        line: 8
+        column: 5
+    - WriteLiteral:
+        escape: false
+        text: "</div>"
+      locator:
+        filename: inp
+        line: 7
+        column: 1
diff --git a/hornbeam_ir/src/snapshots/hornbeam_ir__ast_to_ir__tests__pull_out_entrypoints.snap b/hornbeam_ir/src/snapshots/hornbeam_ir__ast_to_ir__tests__pull_out_entrypoints.snap
index 12dc799..4fd27c9 100644
--- a/hornbeam_ir/src/snapshots/hornbeam_ir__ast_to_ir__tests__pull_out_entrypoints.snap
+++ b/hornbeam_ir/src/snapshots/hornbeam_ir__ast_to_ir__tests__pull_out_entrypoints.snap
@@ -3,67 +3,74 @@ source: hornbeam_ir/src/ast_to_ir.rs
 expression: "pull_out_entrypoints(template, \"TemplateName\").unwrap()"
 ---
 TemplateName:
-  - HtmlElement:
-      name: div
-      children:
-        - DefineFragment:
-            name: TemplateName__Frag1
-            blocks: []
-            loc:
-              filename: inp
-              line: 3
-              column: 5
-        - DefineFragment:
-            name: TemplateName__Footer
-            blocks: []
-            loc:
-              filename: inp
-              line: 9
-              column: 5
-      classes: []
-      dom_id: ~
-      attributes: {}
-      loc:
-        filename: inp
-        line: 2
-        column: 1
+  param_defs: ~
+  blocks:
+    - HtmlElement:
+        name: div
+        children:
+          - DefineFragment:
+              name: TemplateName__Frag1
+              blocks: []
+              loc:
+                filename: inp
+                line: 3
+                column: 5
+          - DefineFragment:
+              name: TemplateName__Footer
+              blocks: []
+              loc:
+                filename: inp
+                line: 9
+                column: 5
+        classes: []
+        dom_id: ~
+        attributes: {}
+        loc:
+          filename: inp
+          line: 2
+          column: 1
 TemplateName__Footer:
-  - Text:
-      pieces:
-        - Literal: Or even adjacent ones
+  param_defs: ~
+  blocks:
+    - Text:
+        pieces:
+          - Literal: Or even adjacent ones
 TemplateName__Frag1:
-  - HtmlElement:
-      name: span
-      children:
-        - Text:
-            pieces:
-              - Literal: This is a fragment!!
-      classes: []
-      dom_id: ~
-      attributes: {}
-      loc:
-        filename: inp
-        line: 4
-        column: 9
-  - DefineFragment:
-      name: TemplateName__Frag2
-      blocks: []
-      loc:
-        filename: inp
-        line: 6
-        column: 9
+  param_defs: ~
+  blocks:
+    - HtmlElement:
+        name: span
+        children:
+          - Text:
+              pieces:
+                - Literal: This is a fragment!!
+        classes: []
+        dom_id: ~
+        attributes: {}
+        loc:
+          filename: inp
+          line: 4
+          column: 9
+    - DefineFragment:
+        name: TemplateName__Frag2
+        blocks: []
+        loc:
+          filename: inp
+          line: 6
+          column: 9
 TemplateName__Frag2:
-  - HtmlElement:
-      name: div
-      children:
-        - Text:
-            pieces:
-              - Literal: "There's no problem having nested fragments!"
-      classes: []
-      dom_id: ~
-      attributes: {}
-      loc:
-        filename: inp
-        line: 7
-        column: 13
-
+  param_defs: ~
+  blocks:
+    - HtmlElement:
+        name: div
+        children:
+          - Text:
+              pieces:
+                - Literal: "There's no problem having nested fragments!"
+        classes: []
+        dom_id: ~
+        attributes: {}
+        loc:
+          filename: inp
+          line: 7
+          column: 13