diff --git a/examples/euler13.zr b/examples/euler13.zr
new file mode 100644
index 0000000..27cf2d0
--- /dev/null
+++ b/examples/euler13.zr
@@ -0,0 +1,5 @@
+func main[] : I64
+    37107287533 + 46376937677 + 74324986199 + 91942213363 + 23067588207 + 89261670696 + 28112879812 + 44274228917 + 47451445736 + 70386486105 + 62176457141 + 64906352462 + 92575867718 + 58203565325 + 80181199384 + 35398664372 + 86515506006 + 71693888707 + 54370070576 + 53282654108 + 36123272525 + 45876576172 + 17423706905 + 81142660418 + 51934325451 + 62467221648 + 15732444386 + 55037687525 + 18336384825 + 80386287592 + 78182833757 + 16726320100 + 48403098129 + 87086987551 + 59959406895 + 69793950679 + 41052684708 + 65378607361 + 35829035317 + 94953759765 + 88902802571 + 25267680276 + 36270218540 + 24074486908 + 91430288197 + 34413065578 + 23053081172 + 11487696932 + 63783299490 + 67720186971 + 95548255300 + 76085327132 + 37774242535 + 23701913275 + 29798860272 + 18495701454 + 38298203783 + 34829543829 + 40957953066 + 29746152185 + 41698116222 + 62467957194 + 23189706772 + 86188088225 + 11306739708 + 82959174767 + 97623331044 + 42846280183 + 55121603546 + 32238195734 + 75506164965 + 62177842752 + 32924185707 + 99518671430 + 73267460800 + 76841822524 + 97142617910 + 87783646182 + 10848802521 + 71329612474 + 62184073572 + 66627891981 + 60661826293 + 85786944089 + 66024396409 + 64913982680 + 16730939319 + 94809377245 + 78639167021 + 15368713711 + 40789923115 + 44889911501 + 41503128880 + 81234880673 + 82616570773 + 22918802058 + 77158542502 + 72107838435 + 20849603980 + 53503534226
+    |> I64.to_string()
+    |> substr(0, 10)
+    |> print()
\ No newline at end of file
diff --git a/examples/euler15.zr b/examples/euler15.zr
new file mode 100644
index 0000000..79b8276
--- /dev/null
+++ b/examples/euler15.zr
@@ -0,0 +1,11 @@
+func main[] : I64
+    let n: I64 = 40
+    let r: I64 = 20
+    let out: I64 = 1
+
+    let i: I64 = 1
+    while i < r + 1
+        out = out * (n - (r - i)) / i
+        i = i + 1
+
+    print_i64(out)
\ No newline at end of file
diff --git a/src/codegen_x86_64.rs b/src/codegen_x86_64.rs
index 599f066..9019f66 100644
--- a/src/codegen_x86_64.rs
+++ b/src/codegen_x86_64.rs
@@ -58,7 +58,7 @@ macro_rules! emit {
 }
 
 static REGISTERS: [&str; 6] = ["rdi", "rsi", "rdx", "rcx", "r8", "r9"];
-static TYPES: [&str; 3] = ["I64", "String", "Bool"];
+static TYPES: [&str; 5] = ["I64", "String", "Bool", "Ptr", "Char"];
 
 pub struct CodegenX86_64 {
     output: String,
@@ -85,6 +85,7 @@ impl CodegenX86_64 {
     pub fn get_output(&self) -> String {
         format!(
             "section .data
+    SASSERT db \"assertion failed on line %d\",10,0
 {}{}",
             self.data_section, self.output
         )
@@ -104,38 +105,17 @@ extern strcmp
 extern strcat
 extern strcpy
 extern strdup
+extern strlcpy
 extern puts
+extern fopen
+extern fseek
+extern ftell
+extern fread
+extern rewind
 extern system
 extern exit
 copystr equ strdup
 
-strrev:
-    push    r14
-    push    rbx
-    push    rax
-    mov     rbx, rdi
-    call    strlen
-    mov     r14, rax
-    lea     rdi, [rax + 1]
-    call    malloc
-    mov     rcx, rax
-    mov     rsi, r14
-    mov     rdx, r14
-.strrev.1:
-    sub     rdx, 1
-    jb      .strrev.2
-    mov     sil, byte [rbx + rsi - 1]
-    mov     byte [rcx], sil
-    inc     rcx
-    mov     rsi, rdx
-    jmp     .strrev.1
-.strrev.2:
-    mov     byte [rax + r14], 0
-    add     rsp, 8
-    pop     rbx
-    pop     r14
-    ret
-
 isqrt:
     xor     rax, rax
     mov     rcx, 1
@@ -170,6 +150,10 @@ isqrt:
 nth:
     movzx rax, byte [rdi + rsi]
     ret
+
+set:
+    mov [rdi + rsi], dl
+    ret
 ",
         );
         Ok(())
@@ -275,6 +259,18 @@ nth:
                 emit!(&mut self.output, "    pop rbp");
                 emit!(&mut self.output, "    ret");
             }
+            Stmt::Assert { keyword, value } => {
+                self.compile_expr(env, value)?;
+                let skip_label = self.label();
+                emit!(&mut self.output, "    test rax, rax");
+                emit!(&mut self.output, "    jne {}", skip_label);
+                emit!(&mut self.output, "    mov rdi, SASSERT");
+                emit!(&mut self.output, "    mov rsi, {}", keyword.loc.line);
+                emit!(&mut self.output, "    call printf");
+                emit!(&mut self.output, "    mov rdi, 1");
+                emit!(&mut self.output, "    call exit");
+                emit!(&mut self.output, "{}:", skip_label);
+            }
         }
         Ok(())
     }
@@ -431,6 +427,7 @@ nth:
                 paren,
                 args,
             } => {
+                // TODO: in function calls like a(1, b(2, 3)) the first argument will get overwritten when calling b
                 let callee = match *callee {
                     Expr::Variable(name) => name.lexeme,
                     _ => return error!(&paren.loc, "tried to call a non-constant expression"),
diff --git a/src/parser.rs b/src/parser.rs
index 9dfcdcd..914bfae 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -31,6 +31,10 @@ pub enum Stmt {
         body: Box<Stmt>,
     },
     Return(Expr),
+    Assert {
+        keyword: Token,
+        value: Expr,
+    },
 }
 
 #[derive(Debug, Clone)]
@@ -164,16 +168,17 @@ impl Parser {
         } else if self.match_token(&[TokenType::KeywordWhile]) {
             self.while_statement()
         } else if self.match_token(&[TokenType::KeywordReturn]) {
-            self.return_statement()
+            Ok(Stmt::Return(self.expression()?))
+        } else if self.match_token(&[TokenType::KeywordAssert]) {
+            Ok(Stmt::Assert {
+                keyword: self.previous().clone(),
+                value: self.expression()?,
+            })
         } else {
             Ok(Stmt::Expression(self.expression()?))
         }
     }
 
-    fn return_statement(&mut self) -> Result<Stmt, ZernError> {
-        Ok(Stmt::Return(self.expression()?))
-    }
-
     fn if_statement(&mut self) -> Result<Stmt, ZernError> {
         let condition = self.expression()?;
         let then_branch = self.block()?;
diff --git a/src/std.zr b/src/std.zr
index 6175172..1c8e8e4 100644
--- a/src/std.zr
+++ b/src/std.zr
@@ -1,3 +1,7 @@
+func panic[msg: String] : I64
+    printf("PANIC: %s\n", msg)
+    exit(1)
+
 func print[x: String] : I64
     printf("%s\n", x)
 
@@ -10,7 +14,39 @@ func concat[a: String, b: String] : String
     strcat(c, b)
     return c
 
-func Char.to_i64[c: I64]: I64
+func substr[s: String, start: I64, length: I64] : String
+    let out: String = malloc(length + 1)
+    strlcpy(out, s + start, length + 1)
+    return out
+
+func strrev[s: String] : String
+    let len: I64 = strlen(s)
+    let out: String = malloc(len + 1)
+
+    let i: I64 = 0
+    while i < len
+        let c: Char = nth(s, len - i - 1)
+        set(out, i, c)
+        i = i + 1
+    set(out, len, 0)
+    return out
+
+func IO.read_file[path: String]: String
+    let file: Ptr = fopen(path, "rb")
+    if !file
+        panic("failed to open file")
+
+    fseek(file, 0, 2)
+    let size: I64 = ftell(file)
+    rewind(file)
+
+    let buffer: String = malloc(size + 1)
+
+    let n: I64 = fread(buffer, 1, size, file)
+    set(buffer, n, 0)
+    return buffer
+
+func Char.to_i64[c: Char]: I64
     return c - 48
 
 func I64.to_string[n: I64] : String
diff --git a/src/tokenizer.rs b/src/tokenizer.rs
index c638a2d..cf50c1f 100644
--- a/src/tokenizer.rs
+++ b/src/tokenizer.rs
@@ -39,6 +39,7 @@ pub enum TokenType {
     KeywordWhile,
     KeywordFunc,
     KeywordReturn,
+    KeywordAssert,
 
     Indent,
     Dedent,
@@ -311,6 +312,7 @@ impl Tokenizer {
             "while" => TokenType::KeywordWhile,
             "func" => TokenType::KeywordFunc,
             "return" => TokenType::KeywordReturn,
+            "assert" => TokenType::KeywordAssert,
             "true" => TokenType::True,
             "false" => TokenType::False,
             _ => TokenType::Identifier,