diff --git a/.gitignore b/.gitignore
index fe932d1..a2efa16 100644
--- a/.gitignore
+++ b/.gitignore
@@ -33,4 +33,6 @@ Thumbs.db
 Desktop.ini
 .DS_Store
 
-Nub.Lang/Nub.Lang/Output/out*
\ No newline at end of file
+Nub.Lang/Nub.Lang/Output/*.o
+Nub.Lang/Nub.Lang/Output/out
+Nub.Lang/Nub.Lang/Output/out.asm
diff --git a/Nub.Lang/Nub.Lang/Branching/BranchChecker.cs b/Nub.Lang/Nub.Lang/Branching/BranchChecker.cs
index 8954360..5223c46 100644
--- a/Nub.Lang/Nub.Lang/Branching/BranchChecker.cs
+++ b/Nub.Lang/Nub.Lang/Branching/BranchChecker.cs
@@ -13,7 +13,7 @@ public class BranchChecker
 
     public void Check()
     {
-        foreach (var funcDefinition in _definitions.OfType<FuncDefinitionNode>())
+        foreach (var funcDefinition in _definitions.OfType<LocalFuncDefinitionNode>())
         {
             if (funcDefinition.ReturnType.HasValue)
             {
diff --git a/Nub.Lang/Nub.Lang/Generation/Generator.cs b/Nub.Lang/Nub.Lang/Generation/Generator.cs
index f8af200..002764b 100644
--- a/Nub.Lang/Nub.Lang/Generation/Generator.cs
+++ b/Nub.Lang/Nub.Lang/Generation/Generator.cs
@@ -11,6 +11,7 @@ public class Generator
     private readonly SymbolTable _symbolTable;
     private readonly StringBuilder _builder;
     private readonly Dictionary<string, string> _strings;
+    private readonly HashSet<string> _externFuncDefinitions;
     private int _stringIndex;
     
     public Generator(IReadOnlyCollection<DefinitionNode> definitions)
@@ -18,8 +19,15 @@ public class Generator
         _strings = [];
         _definitions = definitions;
         _builder = new StringBuilder();
+        _externFuncDefinitions = ["strcmp"];
         _symbolTable = new SymbolTable(definitions.OfType<GlobalVariableDefinitionNode>().ToList());
-        foreach (var funcDefinitionNode in definitions.OfType<FuncDefinitionNode>())
+        
+        foreach (var funcDefinitionNode in definitions.OfType<ExternFuncDefinitionNode>())
+        {
+            _symbolTable.DefineFunc(funcDefinitionNode);
+            _externFuncDefinitions.Add(_symbolTable.ResolveExternFunc(funcDefinitionNode.Name, funcDefinitionNode.Parameters.Select(p => p.Type).ToList()).StartLabel);
+        }
+        foreach (var funcDefinitionNode in definitions.OfType<LocalFuncDefinitionNode>())
         {
             _symbolTable.DefineFunc(funcDefinitionNode);
         }
@@ -29,6 +37,11 @@ public class Generator
     {
         _builder.AppendLine("global _start");
         
+        foreach (var externFuncDefinition in _externFuncDefinitions)
+        {
+            _builder.AppendLine($"extern {externFuncDefinition}");
+        }
+        
         _builder.AppendLine();
         _builder.AppendLine("section .bss");
         foreach (var globalVariable in _definitions.OfType<GlobalVariableDefinitionNode>())
@@ -41,7 +54,7 @@ public class Generator
         _builder.AppendLine("section .text");
         _builder.AppendLine("_start:");
         
-        var main = _symbolTable.ResolveFunc(Entrypoint, []);
+        var main = _symbolTable.ResolveLocalFunc(Entrypoint, []);
         
         _builder.AppendLine("    ; Initialize global variables");
         foreach (var globalVariable in _definitions.OfType<GlobalVariableDefinitionNode>())
@@ -56,56 +69,17 @@ public class Generator
         _builder.AppendLine($"    call {main.StartLabel}");
 
         _builder.AppendLine();
-        _builder.AppendLine("    ; Exit with status code 0");
+        _builder.AppendLine(main.ReturnType.HasValue
+            ? "    mov rdi, rax ; Exit with return value of entrypoint"
+            : "    mov rdi, 0 ; Exit with default status code 0");
         _builder.AppendLine("    mov rax, 60");
-        _builder.AppendLine("    mov rdi, 0");
         _builder.AppendLine("    syscall");
 
-        foreach (var funcDefinition in _definitions.OfType<FuncDefinitionNode>())
+        foreach (var funcDefinition in _definitions.OfType<LocalFuncDefinitionNode>())
         {
             _builder.AppendLine();
             GenerateFuncDefinition(funcDefinition);
         }
-
-        _builder.AppendLine("""
-                            
-                            ; https://tuttlem.github.io/2013/01/08/strlen-implementation-in-nasm.html
-                            strlen:
-                                push rcx            ; save and clear out counter
-                                xor rcx, rcx
-                            strlen_next:
-                                cmp [rdi], byte 0   ; null byte yet?
-                                jz strlen_null      ; yes, get out
-                                inc rcx             ; char is ok, count it
-                                inc rdi             ; move to next char
-                                jmp strlen_next     ; process again
-                            strlen_null:
-                                mov rax, rcx        ; rcx = the length (put in rax)
-                                pop rcx             ; restore rcx
-                                ret                 ; get out
-                            """);
-
-        _builder.AppendLine("""
-                            
-                            strcmp:
-                                xor rdx, rdx
-                            strcmp_loop:
-                                mov al, [rsi + rdx]
-                                mov bl, [rdi + rdx]
-                                inc rdx
-                                cmp al, bl
-                                jne strcmp_not_equal
-                                cmp al, 0
-                                je strcmp_equal
-                                jmp strcmp_loop
-                            strcmp_not_equal:
-                                mov rax, 0
-                                ret
-                            strcmp_equal:
-                                mov rax, 1
-                                ret
-                            """);
-        
         
         _builder.AppendLine();
         _builder.AppendLine("section .data");
@@ -117,9 +91,10 @@ public class Generator
         return _builder.ToString();
     }
 
-    private void GenerateFuncDefinition(FuncDefinitionNode node)
+    private void GenerateFuncDefinition(LocalFuncDefinitionNode node)
     {
-        var func = _symbolTable.ResolveFunc(node.Name, node.Parameters.Select(p => p.Type).ToList());
+        var func = _symbolTable.ResolveLocalFunc(node.Name, node.Parameters.Select(p => p.Type).ToList());
+        
         _builder.AppendLine($"; {node.ToString()}");
         _builder.AppendLine($"{func.StartLabel}:");
         _builder.AppendLine("    ; Set up stack frame");
@@ -153,8 +128,8 @@ public class Generator
         _builder.AppendLine("    pop rbp");
         _builder.AppendLine("    ret");
     }
-
-    private void GenerateBlock(BlockNode block, Func func)
+    
+    private void GenerateBlock(BlockNode block, LocalFunc func)
     {
         foreach (var statement in block.Statements)
         {
@@ -162,7 +137,7 @@ public class Generator
         }
     }
 
-    private void GenerateStatement(StatementNode statement, Func func)
+    private void GenerateStatement(StatementNode statement, LocalFunc func)
     {
         switch (statement)
         {
@@ -170,13 +145,13 @@ public class Generator
                 GenerateFuncCall(funcCallStatement.FuncCall, func);
                 break;
             case ReturnNode @return:
-                GenerateReturn(func, @return);
+                GenerateReturn(@return, func);
                 break;
             case SyscallStatementNode syscallStatement:
                 GenerateSyscall(syscallStatement.Syscall, func);
                 break;
             case VariableAssignmentNode variableAssignment:
-                GenerateVariableAssignment(func, variableAssignment);
+                GenerateVariableAssignment(variableAssignment, func);
                 break;
             case VariableReassignmentNode variableReassignment:
                 GenerateVariableReassignment(variableReassignment, func);
@@ -186,7 +161,7 @@ public class Generator
         }
     }
 
-    private void GenerateReturn(Func func, ReturnNode @return)
+    private void GenerateReturn(ReturnNode @return, LocalFunc func)
     {
         if (@return.Value.HasValue)
         {
@@ -196,21 +171,21 @@ public class Generator
         _builder.AppendLine($"    jmp {func.EndLabel}");
     }
 
-    private void GenerateVariableAssignment(Func func, VariableAssignmentNode variableAssignment)
+    private void GenerateVariableAssignment(VariableAssignmentNode variableAssignment, LocalFunc func)
     {
         var variable = func.ResolveLocalVariable(variableAssignment.Name);
         GenerateExpression(variableAssignment.Value, func);
         _builder.AppendLine($"    mov [rbp - {variable.Offset}], rax");
     }
 
-    private void GenerateVariableReassignment(VariableReassignmentNode variableReassignment, Func func)
+    private void GenerateVariableReassignment(VariableReassignmentNode variableReassignment, LocalFunc func)
     {
         var variable = func.ResolveLocalVariable(variableReassignment.Name);
         GenerateExpression(variableReassignment.Value, func);
         _builder.AppendLine($"    mov [rbp - {variable.Offset}], rax");
     }
 
-    private void GenerateExpression(ExpressionNode expression, Func func)
+    private void GenerateExpression(ExpressionNode expression, LocalFunc func)
     {
         switch (expression)
         {
@@ -224,10 +199,7 @@ public class Generator
                 GenerateIdentifier(identifier, func);
                 break;
             case LiteralNode literal:
-                GenerateLiteral(literal, func);
-                break;
-            case StrlenNode strlen:
-                GenerateStrlen(strlen, func);
+                GenerateLiteral(literal);
                 break;
             case SyscallExpressionNode syscallExpression:
                 GenerateSyscall(syscallExpression.Syscall, func);
@@ -237,7 +209,7 @@ public class Generator
         }
     }
 
-    private void GenerateBinaryExpression(BinaryExpressionNode binaryExpression, Func func)
+    private void GenerateBinaryExpression(BinaryExpressionNode binaryExpression, LocalFunc func)
     {
         GenerateExpression(binaryExpression.Left, func);
         _builder.AppendLine("    push rax");
@@ -396,7 +368,7 @@ public class Generator
         }
     }
 
-    private void GenerateIdentifier(IdentifierNode identifier, Func func)
+    private void GenerateIdentifier(IdentifierNode identifier, LocalFunc func)
     {
         var variable = func.ResolveVariable(identifier.Identifier);
 
@@ -417,7 +389,7 @@ public class Generator
         }
     }
 
-    private void GenerateLiteral(LiteralNode literal, Func func)
+    private void GenerateLiteral(LiteralNode literal)
     {
         switch (literal.Type)
         {
@@ -459,14 +431,7 @@ public class Generator
         }
     }
 
-    private void GenerateStrlen(StrlenNode strlen, Func func)
-    {
-        GenerateExpression(strlen.String, func);
-        _builder.AppendLine("    mov rdi, rax");
-        _builder.AppendLine("    call strlen");
-    }
-
-    private void GenerateFuncCall(FuncCall funcCall, Func func)
+    private void GenerateFuncCall(FuncCall funcCall, LocalFunc func)
     {
         var symbol = _symbolTable.ResolveFunc(funcCall.Name, funcCall.Parameters.Select(p => p.Type).ToList());
         string[] registers = ["rdi", "rsi", "rdx", "rcx", "r8", "r9"];
@@ -492,7 +457,7 @@ public class Generator
         }
     }
     
-    private void GenerateSyscall(Syscall syscall, Func func)
+    private void GenerateSyscall(Syscall syscall, LocalFunc func)
     {
         string[] registers = ["rax", "rdi", "rsi", "rdx", "r10", "r8", "r9"];
 
diff --git a/Nub.Lang/Nub.Lang/Generation/SymbolTable.cs b/Nub.Lang/Nub.Lang/Generation/SymbolTable.cs
index 8dff559..61ad307 100644
--- a/Nub.Lang/Nub.Lang/Generation/SymbolTable.cs
+++ b/Nub.Lang/Nub.Lang/Generation/SymbolTable.cs
@@ -5,7 +5,7 @@ namespace Nub.Lang.Generation;
 
 public class SymbolTable
 {
-    private readonly List<Func> _functions = [];
+    private readonly List<Func> _funcDefinitions = [];
     private readonly List<GlobalVariable> _globalVariables = [];
     private int _labelIndex;
 
@@ -19,26 +19,30 @@ public class SymbolTable
         }
     }
     
-    public void DefineFunc(FuncDefinitionNode funcDefinition)
+    public void DefineFunc(ExternFuncDefinitionNode externFuncDefinition)
+    {
+        _funcDefinitions.Add(new ExternFunc(externFuncDefinition.Name, externFuncDefinition.Name, externFuncDefinition.Parameters, externFuncDefinition.ReturnType));
+    }
+    
+    public void DefineFunc(LocalFuncDefinitionNode localFuncDefinition)
     {
         var startLabel = $"func{++_labelIndex}";
-        var endLabel = $"endfunc{_labelIndex}";
-        var localVariables = ResolveFunctionVariables(funcDefinition);
-        _functions.Add(new Func(startLabel, endLabel, funcDefinition.Name, funcDefinition.Parameters, funcDefinition.ReturnType, _globalVariables.Concat<Variable>(localVariables.Variables).ToList(), localVariables.StackSize));
+        var endLabel = $"func_end{_labelIndex}";
+        _funcDefinitions.Add(new LocalFunc(localFuncDefinition.Name, startLabel, endLabel, localFuncDefinition.Parameters, localFuncDefinition.ReturnType, _globalVariables.Concat<Variable>(ResolveFuncVariables(localFuncDefinition)).ToList()));
     }
-
-    private (int StackSize, List<LocalVariable> Variables) ResolveFunctionVariables(FuncDefinitionNode funcDefinition)
+    
+    private static List<LocalVariable> ResolveFuncVariables(LocalFuncDefinitionNode localFuncDefinition)
     {
         var offset = 0;
         List<LocalVariable> variables = [];
 
-        foreach (var parameter in funcDefinition.Parameters)
+        foreach (var parameter in localFuncDefinition.Parameters)
         {
             offset += 8;
             variables.Add(new LocalVariable(parameter.Name, parameter.Type, offset));
         }
         
-        foreach (var statement in funcDefinition.Body.Statements)
+        foreach (var statement in localFuncDefinition.Body.Statements)
         {
             if (statement is VariableAssignmentNode variableAssignment)
             {
@@ -47,12 +51,12 @@ public class SymbolTable
             }
         }
 
-        return (offset, variables);
+        return variables;
     }
     
     public Func ResolveFunc(string name, IReadOnlyCollection<Type> parameterTypes)
     {
-        var func = _functions.FirstOrDefault(f => f.Name == name && f.Parameters.Count == parameterTypes.Count && f.Parameters.Where((p, i) => p.Type == parameterTypes.ElementAt(i)).Count() == parameterTypes.Count);
+        var func = _funcDefinitions.FirstOrDefault(f => f.Name == name && f.Parameters.Count == parameterTypes.Count && f.Parameters.Where((p, i) => p.Type == parameterTypes.ElementAt(i)).Count() == parameterTypes.Count);
         if (func == null)
         {
             throw new Exception($"Func {name}({string.Join(", ", parameterTypes)}) is not defined");
@@ -61,6 +65,26 @@ public class SymbolTable
         return func;
     }
     
+    public LocalFunc ResolveLocalFunc(string name, IReadOnlyCollection<Type> parameterTypes)
+    {
+        var func = ResolveFunc(name, parameterTypes);
+        if (func is not LocalFunc localFunc)
+        {
+            throw new Exception($"Func {func} is not a local func");
+        }
+        return localFunc;
+    }
+    
+    public ExternFunc ResolveExternFunc(string name, IReadOnlyCollection<Type> parameterTypes)
+    {
+        var func = ResolveFunc(name, parameterTypes);
+        if (func is not ExternFunc externFunc)
+        {
+            throw new Exception($"Func {func} is not an extern func");
+        }
+        return externFunc;
+    }
+    
     public GlobalVariable ResolveGlobalVariable(string name)
     {
         var variable = _globalVariables.FirstOrDefault(v => v.Name == name);
@@ -89,15 +113,40 @@ public class GlobalVariable(string name, Type type, string identifier) : Variabl
     public string Identifier { get; } = identifier;
 }
 
-public class Func(string startLabel, string endLabel, string name, IReadOnlyCollection<FuncParameter> parameters, Optional<Type> returnType, IReadOnlyCollection<Variable> variables, int stackAllocation)
+public abstract class Func
 {
-    public string StartLabel { get; } = startLabel;
-    public string EndLabel { get; } = endLabel;
-    public string Name { get; } = name;
-    public IReadOnlyCollection<FuncParameter> Parameters { get; } = parameters;
-    public Optional<Type> ReturnType { get; } = returnType;
-    public IReadOnlyCollection<Variable> Variables { get; } = variables;
-    public int StackAllocation { get; } = stackAllocation;
+    public Func(string name, string startLabel, IReadOnlyCollection<FuncParameter> parameters, Optional<Type> returnType)
+    {
+        Name = name;
+        Parameters = parameters;
+        ReturnType = returnType;
+        StartLabel = startLabel;
+    }
+
+    public string Name { get; }
+    public string StartLabel { get; }
+    public IReadOnlyCollection<FuncParameter> Parameters { get; }
+    public Optional<Type> ReturnType { get; }
+}
+
+public class ExternFunc : Func
+{
+    public ExternFunc(string name, string startLabel, IReadOnlyCollection<FuncParameter> parameters, Optional<Type> returnType) : base(name, startLabel, parameters, returnType)
+    {
+    }
+}
+
+public class LocalFunc : Func
+{
+    public LocalFunc(string name, string startLabel, string endLabel, IReadOnlyCollection<FuncParameter> parameters, Optional<Type> returnType, IReadOnlyCollection<Variable> variables) : base(name, startLabel, parameters, returnType)
+    {
+        EndLabel = endLabel;
+        Variables = variables;
+    }
+
+    public string EndLabel { get; }
+    public IReadOnlyCollection<Variable> Variables { get; }
+    public int StackAllocation => Variables.OfType<LocalVariable>().Sum(variable => variable.Offset);
 
     public Variable ResolveVariable(string name)
     {
diff --git a/Nub.Lang/Nub.Lang/Input/program.nub b/Nub.Lang/Nub.Lang/Input/program.nub
index eaaf715..599b0b5 100644
--- a/Nub.Lang/Nub.Lang/Input/program.nub
+++ b/Nub.Lang/Nub.Lang/Input/program.nub
@@ -1,3 +1,15 @@
+let SYS_WRITE = 1;
+let STD_IN = 0;
+let STD_OUT = 1;
+let STD_ERR = 2;
+
 func main() {
-    syscall(60, 5 + 3 * 5);
-}
\ No newline at end of file
+    print("test\n");
+}
+
+func print(msg: String) {
+    syscall(SYS_WRITE, STD_OUT, msg, strlen(msg));
+}
+
+extern func strlen(msg: String): int64;
+extern func strcmp(a: String, b: String): bool;
diff --git a/Nub.Lang/Nub.Lang/Lexing/Lexer.cs b/Nub.Lang/Nub.Lang/Lexing/Lexer.cs
index 0f391ec..15befca 100644
--- a/Nub.Lang/Nub.Lang/Lexing/Lexer.cs
+++ b/Nub.Lang/Nub.Lang/Lexing/Lexer.cs
@@ -7,6 +7,7 @@ public class Lexer
     private static readonly Dictionary<string, Symbol> Keywords = new()
     { 
         ["func"] = Symbol.Func,
+        ["extern"] = Symbol.Extern,
         ["return"] = Symbol.Return,
         ["let"] = Symbol.Let,
     };
diff --git a/Nub.Lang/Nub.Lang/Lexing/SymbolToken.cs b/Nub.Lang/Nub.Lang/Lexing/SymbolToken.cs
index 2910f2f..e67c07f 100644
--- a/Nub.Lang/Nub.Lang/Lexing/SymbolToken.cs
+++ b/Nub.Lang/Nub.Lang/Lexing/SymbolToken.cs
@@ -8,6 +8,7 @@ public class SymbolToken(Symbol symbol) : Token
 public enum Symbol
 {
     Whitespace,
+    Extern,
     Func,
     Return,
     Let,
diff --git a/Nub.Lang/Nub.Lang/Output/build.sh b/Nub.Lang/Nub.Lang/Output/build.sh
new file mode 100644
index 0000000..3607710
--- /dev/null
+++ b/Nub.Lang/Nub.Lang/Output/build.sh
@@ -0,0 +1,6 @@
+#!/bin/sh
+nasm -g -felf64 out.asm -o out.o
+nasm -g -felf64 core/strlen.asm -o strlen.o
+nasm -g -felf64 core/strcmp.asm -o strcmp.o
+
+ld -o out out.o strlen.o strcmp.o
diff --git a/Nub.Lang/Nub.Lang/Output/core/strcmp.asm b/Nub.Lang/Nub.Lang/Output/core/strcmp.asm
new file mode 100644
index 0000000..b5035e6
--- /dev/null
+++ b/Nub.Lang/Nub.Lang/Output/core/strcmp.asm
@@ -0,0 +1,20 @@
+global strlen
+section .text
+
+strcmp:
+    xor rdx, rdx
+.loop:
+    mov al, [rsi + rdx]
+    mov bl, [rdi + rdx]
+    inc rdx
+    cmp al, bl
+    jne .not_equal
+    cmp al, 0
+    je .equal
+    jmp .loop
+.not_equal:
+    mov rax, 0
+    ret
+.equal:
+    mov rax, 1
+    ret
\ No newline at end of file
diff --git a/Nub.Lang/Nub.Lang/Output/core/strlen.asm b/Nub.Lang/Nub.Lang/Output/core/strlen.asm
new file mode 100644
index 0000000..dcd0a3c
--- /dev/null
+++ b/Nub.Lang/Nub.Lang/Output/core/strlen.asm
@@ -0,0 +1,13 @@
+global strlen
+section .text
+
+strlen:
+    xor rax, rax
+.loop:
+    cmp byte [rdi], 0
+    jz .done
+    inc rax
+    inc rdi
+    jmp .loop
+.done:
+    ret
\ No newline at end of file
diff --git a/Nub.Lang/Nub.Lang/Output/debug.sh b/Nub.Lang/Nub.Lang/Output/debug.sh
index ad569b6..189ee44 100755
--- a/Nub.Lang/Nub.Lang/Output/debug.sh
+++ b/Nub.Lang/Nub.Lang/Output/debug.sh
@@ -1,9 +1,3 @@
-#!/bin/bash
-
-nasm -g -felf64 out.asm -o out.o
-ld out.o -o out
-
+#!/bin/sh
+./build.sh
 gdb -tui out
-
-rm out.o
-rm out
\ No newline at end of file
diff --git a/Nub.Lang/Nub.Lang/Output/run.sh b/Nub.Lang/Nub.Lang/Output/run.sh
index 7788169..b5a3090 100755
--- a/Nub.Lang/Nub.Lang/Output/run.sh
+++ b/Nub.Lang/Nub.Lang/Output/run.sh
@@ -1,10 +1,4 @@
-#!/bin/bash
-
-nasm -g -felf64 out.asm -o out.o
-ld out.o -o out
-
+#!/bin/sh
+./build.sh
 ./out
 echo "Process exited with status code $?"
-
-rm out.o
-rm out
\ No newline at end of file
diff --git a/Nub.Lang/Nub.Lang/Parsing/ExternFuncDefinitionNode.cs b/Nub.Lang/Nub.Lang/Parsing/ExternFuncDefinitionNode.cs
new file mode 100644
index 0000000..e4fd036
--- /dev/null
+++ b/Nub.Lang/Nub.Lang/Parsing/ExternFuncDefinitionNode.cs
@@ -0,0 +1,12 @@
+using Nub.Core;
+
+namespace Nub.Lang.Parsing;
+
+public class ExternFuncDefinitionNode(string name, IReadOnlyCollection<FuncParameter> parameters, Optional<Type> returnType) : DefinitionNode
+{
+    public string Name { get; } = name;
+    public IReadOnlyCollection<FuncParameter> Parameters { get; } = parameters;
+    public Optional<Type> ReturnType { get; } = returnType;
+
+    public override string ToString() => $"{Name}({string.Join(", ", Parameters.Select(p => p.ToString()))}){(ReturnType.HasValue ? ": " + ReturnType.Value : "")}";
+}
\ No newline at end of file
diff --git a/Nub.Lang/Nub.Lang/Parsing/FuncDefinitionNode.cs b/Nub.Lang/Nub.Lang/Parsing/LocalFuncDefinitionNode.cs
similarity index 73%
rename from Nub.Lang/Nub.Lang/Parsing/FuncDefinitionNode.cs
rename to Nub.Lang/Nub.Lang/Parsing/LocalFuncDefinitionNode.cs
index 65cdb0d..b2c0e45 100644
--- a/Nub.Lang/Nub.Lang/Parsing/FuncDefinitionNode.cs
+++ b/Nub.Lang/Nub.Lang/Parsing/LocalFuncDefinitionNode.cs
@@ -2,7 +2,7 @@
 
 namespace Nub.Lang.Parsing;
 
-public class FuncDefinitionNode(string name, IReadOnlyCollection<FuncParameter> parameters, BlockNode body, Optional<Type> returnType) : DefinitionNode
+public class LocalFuncDefinitionNode(string name, IReadOnlyCollection<FuncParameter> parameters, BlockNode body, Optional<Type> returnType) : DefinitionNode
 {
     public string Name { get; } = name;
     public IReadOnlyCollection<FuncParameter> Parameters { get; } = parameters;
diff --git a/Nub.Lang/Nub.Lang/Parsing/Parser.cs b/Nub.Lang/Nub.Lang/Parsing/Parser.cs
index 1cafe69..0f13196 100644
--- a/Nub.Lang/Nub.Lang/Parsing/Parser.cs
+++ b/Nub.Lang/Nub.Lang/Parsing/Parser.cs
@@ -33,6 +33,7 @@ public class Parser
         {
             Symbol.Let => ParseGlobalVariableDefinition(),
             Symbol.Func => ParseFuncDefinition(),
+            Symbol.Extern => ParseExternFuncDefinition(),
             _ => throw new Exception("Unexpected symbol: " + keyword.Symbol)
         };
     }
@@ -47,7 +48,7 @@ public class Parser
         return new GlobalVariableDefinitionNode(name.Value, value);
     }
 
-    private FuncDefinitionNode ParseFuncDefinition()
+    private LocalFuncDefinitionNode ParseFuncDefinition()
     {
         var name = ExpectIdentifier();
         List<FuncParameter> parameters = [];
@@ -69,7 +70,33 @@ public class Parser
 
         var body = ParseBlock();
 
-        return new FuncDefinitionNode(name.Value, parameters, body, returnType);
+        return new LocalFuncDefinitionNode(name.Value, parameters, body, returnType);
+    }
+
+    private ExternFuncDefinitionNode ParseExternFuncDefinition()
+    {
+        ExpectSymbol(Symbol.Func);
+        var name = ExpectIdentifier();
+        List<FuncParameter> parameters = [];
+        ExpectSymbol(Symbol.OpenParen);
+        if (!TryExpectSymbol(Symbol.CloseParen))
+        {
+            while (!TryExpectSymbol(Symbol.CloseParen))
+            {
+                parameters.Add(ParseFuncParameter());
+                TryExpectSymbol(Symbol.Comma);
+            }
+        }
+
+        var returnType = Optional<Type>.Empty();
+        if (TryExpectSymbol(Symbol.Colon))
+        {
+            returnType = ParseType();
+        }
+
+        ExpectSymbol(Symbol.Semicolon);
+        
+        return new ExternFuncDefinitionNode(name.Value, parameters, returnType);
     }
 
     private FuncParameter ParseFuncParameter()
@@ -268,11 +295,6 @@ public class Parser
                 return new SyscallExpressionNode(new Syscall(parameters));
             }
 
-            if (identifier.Value == "strlen" && parameters.Count == 1)
-            {
-                return new StrlenNode(parameters[0]);
-            }
-
             return new FuncCallExpressionNode(new FuncCall(identifier.Value, parameters));
         }
         
diff --git a/Nub.Lang/Nub.Lang/Parsing/StrlenNode.cs b/Nub.Lang/Nub.Lang/Parsing/StrlenNode.cs
deleted file mode 100644
index 70244e2..0000000
--- a/Nub.Lang/Nub.Lang/Parsing/StrlenNode.cs
+++ /dev/null
@@ -1,6 +0,0 @@
-namespace Nub.Lang.Parsing;
-
-public class StrlenNode(ExpressionNode @string) : ExpressionNode
-{
-    public ExpressionNode String { get; } = @string;
-}
\ No newline at end of file
diff --git a/Nub.Lang/Nub.Lang/Type.cs b/Nub.Lang/Nub.Lang/Type.cs
index 92b316b..16cef56 100644
--- a/Nub.Lang/Nub.Lang/Type.cs
+++ b/Nub.Lang/Nub.Lang/Type.cs
@@ -18,6 +18,7 @@ public record PrimitiveType : Type
             "bool" => PrimitiveTypeKind.Bool,
             "char" => PrimitiveTypeKind.Char,
             "int64" => PrimitiveTypeKind.Int64,
+            "int32" => PrimitiveTypeKind.Int32,
             _ => throw new ArgumentOutOfRangeException(nameof(value), value, null)
         };
         
diff --git a/Nub.Lang/Nub.Lang/Typing/ExpressionTyper.cs b/Nub.Lang/Nub.Lang/Typing/ExpressionTyper.cs
index 1d9a632..cc00042 100644
--- a/Nub.Lang/Nub.Lang/Typing/ExpressionTyper.cs
+++ b/Nub.Lang/Nub.Lang/Typing/ExpressionTyper.cs
@@ -1,16 +1,35 @@
-using Nub.Lang.Parsing;
+using Nub.Core;
+using Nub.Lang.Parsing;
 
 namespace Nub.Lang.Typing;
 
+public class Func(string name, IReadOnlyCollection<FuncParameter> parameters, Optional<BlockNode> body, Optional<Type> returnType)
+{
+    public string Name { get; } = name;
+    public IReadOnlyCollection<FuncParameter> Parameters { get; } = parameters;
+    public Optional<BlockNode> Body { get; } = body;
+    public Optional<Type> ReturnType { get; } = returnType;
+}
+
 public class ExpressionTyper
 {
-    private readonly IReadOnlyCollection<FuncDefinitionNode> _functions;
+    private readonly IReadOnlyCollection<Func> _functions;
     private readonly IReadOnlyCollection<GlobalVariableDefinitionNode> _variableDefinitions;
     private readonly Stack<Variable> _variables;
 
     public ExpressionTyper(IReadOnlyCollection<DefinitionNode> definitions)
     {
-        _functions = definitions.OfType<FuncDefinitionNode>().ToList();
+        var functions = definitions
+            .OfType<LocalFuncDefinitionNode>()
+            .Select(f => new Func(f.Name, f.Parameters, f.Body, f.ReturnType))
+            .ToList();
+        
+        var externFunctions = definitions
+            .OfType<ExternFuncDefinitionNode>()
+            .Select(f => new Func(f.Name, f.Parameters, Optional<BlockNode>.Empty(), f.ReturnType))
+            .ToList();
+        
+        _functions = functions.Concat(externFunctions).ToList();
         _variableDefinitions = definitions.OfType<GlobalVariableDefinitionNode>().ToList();
         _variables = new Stack<Variable>();
     }
@@ -31,8 +50,12 @@ public class ExpressionTyper
             {
                 _variables.Push(new Variable(parameter.Name, parameter.Type));
             }
-            PopulateBlock(function.Body);
-            for (var i = 0; i < function.Parameters.Count(); i++)
+
+            if (function.Body.HasValue)
+            {
+                PopulateBlock(function.Body.Value);
+            }
+            for (var i = 0; i < function.Parameters.Count; i++)
             {
                 _variables.Pop();
             }
@@ -127,9 +150,6 @@ public class ExpressionTyper
             case LiteralNode literal:
                 PopulateLiteral(literal);
                 break;
-            case StrlenNode strlen:
-                PopulateStrlen(strlen);
-                break;
             case SyscallExpressionNode syscall:
                 PopulateSyscallExpression(syscall);
                 break;
@@ -203,11 +223,6 @@ public class ExpressionTyper
         literal.Type = literal.LiteralType;
     }
 
-    private static void PopulateStrlen(StrlenNode strlen)
-    {
-        strlen.Type = new PrimitiveType(PrimitiveTypeKind.Int64);
-    }
-
     private void PopulateSyscallExpression(SyscallExpressionNode syscall)
     {
         foreach (var parameter in syscall.Syscall.Parameters)