diff --git a/src/std/std.zr b/src/std/std.zr
index 8871ad6..016123e 100644
--- a/src/std/std.zr
+++ b/src/std/std.zr
@@ -249,6 +249,9 @@ func io.printf[..] : void
             else if s[0] == 'c'
                 io.print_char(_var_arg(i) as u8)
                 i += 1
+            else if s[0] == 'f'
+                io.print_f64(_var_arg(i) as i64)
+                i += 1
             else if s[0] == '%'
                 io.print_char('%')
             else if s[0] == 0
@@ -293,6 +296,87 @@ func io.println_i64[x: i64] : void
     x->to_str_buf(s)
     io.println(s as str)
 
+// TODO: fix when we implement f64 params
+func io.print_f64[x: i64] : void
+    s := _stackalloc(64)
+    f64.to_str_buf(x, s)
+    io.print(s as str)
+
+func f64.to_str_buf[x: i64, buf: ptr] : void
+    bits := x
+    sign := (bits >> 63) & 1
+    exp := (bits >> 52) & 0x7ff
+    mant := bits & 0x000fffffffffffff
+
+    if exp == 0x7ff
+        if mant == 0
+            if sign
+                mem.copy("-inf" as ptr, buf, 5)
+            else
+                mem.copy("inf" as ptr, buf, 4)
+        else
+            mem.copy("nan" as ptr, buf, 4)
+        return
+
+    p := 0
+    if sign
+        buf[p] = '-'
+        p += 1
+        bits = bits & 0x7fffffffffffffff
+
+    if exp == 0 && mant == 0
+        buf[p] = '0'
+        buf[p+1] = '.'
+        buf[p+2] = '0'
+        buf[p+3] = 0
+        return
+
+    sig := mant
+    if exp != 0
+        sig = sig | 0x0010000000000000
+
+    shift := exp - 1075
+
+    if shift >= 0
+        (sig << shift)->to_str_buf(buf + p)
+        while buf[p]
+            p += 1
+        buf[p] = '.'
+        buf[p+1] = '0'
+        buf[p+2] = 0
+        return
+
+    ns := -shift
+
+    if ns <= 52
+        int_part := sig >> ns
+        int_part->to_str_buf(buf + p)
+        while buf[p]
+            p += 1
+    else
+        buf[p] = '0'
+        p += 1
+
+    buf[p] = '.'
+    p += 1
+
+    mask := (1 << ns) - 1
+    frac := sig & mask
+
+    for i in 0..6
+        if frac == 0
+            if i > 0
+                break
+            buf[p] = '0'
+            p += 1
+            break
+        frac = frac * 10
+        digit := frac >> ns
+        buf[p] = '0' + digit as u8
+        p += 1
+        frac = frac & mask
+    buf[p] = 0
+
 func io.read_char[] : u8
     c := 0 as u8
     _builtin_syscall(SYS_read, STDIN, ^c, 1)
@@ -394,7 +478,7 @@ func str.format_into[..] : i64
     i := 3
     n := 0
 
-    tmp := _stackalloc(21) as str
+    tmp := _stackalloc(64) as str
 
     while s[0]
         if s[0] == '%'
@@ -426,6 +510,14 @@ func str.format_into[..] : i64
                     buf[n] = _var_arg(i) as u8
                 n += 1
                 i += 1
+            else if s[0] == 'f'
+                // TODO: fix when we implement f64 params
+                f64.to_str_buf(_var_arg(i), tmp as ptr)
+                tmp_len := tmp->len()
+                remaining := size - n - 1
+                mem.copy(tmp as ptr, buf + n, math.min(tmp_len, remaining))
+                n += tmp_len
+                i += 1
             else if s[0] == '%'
                 if n < size - 1
                     buf[n] = '%'
@@ -1080,7 +1172,6 @@ func os.time[] : i64
     out := tv->tv_sec * 1000 + tv->tv_usec / 1000
     return out
 
-// voodoo magic
 func os.run_shell_command[command: str] : i64, bool
     pid := _builtin_syscall(SYS_fork)
     if pid < 0
diff --git a/src/typechecker.rs b/src/typechecker.rs
index caea63e..5c51c25 100644
--- a/src/typechecker.rs
+++ b/src/typechecker.rs
@@ -300,9 +300,6 @@ impl<'a> TypeChecker<'a> {
                         "unrecognized type: ".to_owned() + &return_type
                     );
                 }
-                if return_type == "f64" {
-                    return error!(&return_types[0].loc, "returning f64 not implemented yet");
-                }
 
                 self.current_function_return_type = return_type.clone();
 
@@ -559,13 +556,13 @@ impl<'a> TypeChecker<'a> {
                 Ok(field.field_type.clone())
             }
             ExprKind::Cast { expr, type_name } => {
-                if type_name.lexeme == "f64" {
+                let expr_type = self.typecheck_expr(env, expr)?;
+                if expr_type != "any" && type_name.lexeme == "f64" {
                     return error!(
                         &type_name.loc,
-                        "use _builtin_cvtsi2sd and _builtin_cvttsd2si to cast to f64"
+                        "use _builtin_cvtsi2sd and _builtin_cvttsd2si to cast between integers and f64"
                     );
                 }
-                self.typecheck_expr(env, expr)?;
                 if !self.is_valid_type_name(&type_name.lexeme) {
                     return error!(
                         &type_name.loc,