From 413e267d0ed01c426002ec6488f05a5d7a619e5a Mon Sep 17 00:00:00 2001
From: Himbeer <himbeer@disroot.org>
Date: Wed, 4 Sep 2024 16:03:20 +0200
Subject: Require extern functions to be declared

---
 expression.go     |  1 +
 generate.go       | 14 ++++++++++++++
 generate_error.go | 12 +++++++++---
 lex.go            |  2 ++
 parse.go          | 27 ++++++++++++++++++++-------
 token.go          |  3 +++
 6 files changed, 49 insertions(+), 10 deletions(-)

diff --git a/expression.go b/expression.go
index 99bd4cb..6ab30c2 100644
--- a/expression.go
+++ b/expression.go
@@ -41,6 +41,7 @@ func (l linkage) String() string {
 
 type functionExpr struct {
 	link       linkage
+	external   bool
 	name       string
 	params     *paramExpr
 	returnType string
diff --git a/generate.go b/generate.go
index b8c6ba2..a5ddf7f 100644
--- a/generate.go
+++ b/generate.go
@@ -90,6 +90,11 @@ func generateFunction(function *functionExpr, w io.Writer) error {
 	defer func() { currentFunc = "" }()
 
 	funcs[currentFunc] = struct{}{}
+
+	if function.external {
+		return nil
+	}
+
 	localConsts[currentFunc] = map[string]string{}
 	localMuts[currentFunc] = map[string]struct{}{}
 
@@ -579,6 +584,15 @@ func (n *numberExpr) generate(w io.Writer) (string, error) {
 }
 
 func (c *callExpr) generate(w io.Writer) (string, error) {
+	_, ok := funcs[c.funcName]
+	if !ok {
+		return "", errUndeclared{
+			name:   c.funcName,
+			isFunc: true,
+			line:   c.line(),
+		}
+	}
+
 	ret := allocReg()
 
 	fmt.Fprintf(w, "%s =w call $%s(", ret, c.funcName)
diff --git a/generate_error.go b/generate_error.go
index 8dfd964..e89add2 100644
--- a/generate_error.go
+++ b/generate_error.go
@@ -16,12 +16,18 @@ func (e errAlreadyDeclared) Error() string {
 }
 
 type errUndeclared struct {
-	name string
-	line int
+	name   string
+	isFunc bool
+	line   int
 }
 
 func (e errUndeclared) Error() string {
-	return fmt.Sprintf("%d: undeclared variable %q\n", e.line, e.name)
+	kind := "variable"
+	if e.isFunc {
+		kind = "function"
+	}
+
+	return fmt.Sprintf("%d: undeclared %s %q\n", e.line, kind, e.name)
 }
 
 type errImmutable struct {
diff --git a/lex.go b/lex.go
index f0631da..621ed01 100644
--- a/lex.go
+++ b/lex.go
@@ -304,6 +304,8 @@ func addKeywordOrIdentifier(name string, line int) token {
 	switch name {
 	case "export":
 		return token{kind: export, line: line}
+	case "extern":
+		return token{kind: extern, line: line}
 	case "func":
 		return token{kind: function, line: line}
 	case "return":
diff --git a/parse.go b/parse.go
index a577f09..d2f9990 100644
--- a/parse.go
+++ b/parse.go
@@ -93,21 +93,33 @@ func parseToplevel(toks *tokens, errs chan<- error) expression {
 }
 
 func parseFunction(toks *tokens, errs chan<- error) *functionExpr {
-	var link linkage
-
-	ok, err := toks.match(export)
+	exported, err := toks.match(export)
 	if err != nil {
 		errs <- err
 		return nil
 	}
 	line := toks.current().line
-	if ok {
-		link = exportLinkage
-	} else {
+	var external bool
+	if !exported {
 		toks.unreadToken()
+
+		external, err = toks.match(extern)
+		if err != nil {
+			errs <- err
+			return nil
+		}
+		if !external {
+			toks.unreadToken()
+		}
+	}
+
+	var link linkage
+	switch {
+	case exported:
+		link = exportLinkage
 	}
 
-	ok, err = toks.match(function)
+	ok, err := toks.match(function)
 	if err != nil {
 		errs <- err
 		return nil
@@ -171,6 +183,7 @@ func parseFunction(toks *tokens, errs chan<- error) *functionExpr {
 
 	return &functionExpr{
 		link:       link,
+		external:   external,
 		name:       nameTok.value,
 		params:     params,
 		returnType: returnTypeTok.value,
diff --git a/token.go b/token.go
index 42c8041..b1e2dd8 100644
--- a/token.go
+++ b/token.go
@@ -100,6 +100,8 @@ func (k tokenKind) String() string {
 		return "number"
 	case export:
 		return "export"
+	case extern:
+		return "extern"
 	case function:
 		return "func"
 	case ret:
@@ -161,6 +163,7 @@ const (
 	str
 	number
 	export
+	extern
 	function
 	ret
 	constKeyword
-- 
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