add balance example

examples/balance/Main.roc

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+app Main provides [ rocMain ] imports [ Effect ]
+
+rocMain : Effect.Effect {} as Fx
+rocMain =
+    when List.len (Str.split "hello" "JJJJ there") is
+        _  -> Effect.putLine "Yay"
+

examples/balance/platform/Cargo.lock

@@ -0,0 +1,23 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "host"
+version = "0.1.0"
+dependencies = [
+ "roc_std 0.1.0",
+]
+
+[[package]]
+name = "libc"
+version = "0.2.79"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "roc_std"
+version = "0.1.0"
+dependencies = [
+ "libc 0.2.79 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[metadata]
+"checksum libc 0.2.79 (registry+https://github.com/rust-lang/crates.io-index)" = "2448f6066e80e3bfc792e9c98bf705b4b0fc6e8ef5b43e5889aff0eaa9c58743"

examples/balance/platform/Cargo.toml

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+[package]
+name = "host"
+version = "0.1.0"
+authors = ["Richard Feldman <oss@rtfeldman.com>"]
+edition = "2018"
+
+[lib]
+crate-type = ["staticlib"]
+
+[dependencies]
+roc_std = { path = "../../../roc_std" }
+
+[workspace]

examples/balance/platform/Pkg-Config.roc

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+platform folkertdev/foo
+    provides [ mainForHost ]
+    requires { main : Effect {} }
+    imports []
+    effects Effect
+        {
+            putChar : Int -> Effect {},
+            putLine : Str -> Effect {},
+            getLine : Effect Str
+        }
+
+mainForHost : Effect {} as Fx
+mainForHost = main

examples/balance/platform/host.c

@@ -0,0 +1,7 @@
+#include <stdio.h>
+
+extern int rust_main();
+
+int main() {
+  return rust_main();
+}

examples/balance/platform/src/lib.rs

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+#![allow(non_snake_case)]
+
+use roc_std::alloca;
+use roc_std::RocCallResult;
+use roc_std::RocStr;
+use std::alloc::Layout;
+use std::time::SystemTime;
+
+extern "C" {
+    #[link_name = "Main_rocMain_1_exposed"]
+    fn roc_main(output: *mut u8) -> ();
+
+    #[link_name = "Main_rocMain_1_size"]
+    fn roc_main_size() -> i64;
+
+    #[link_name = "Main_rocMain_1_Fx_caller"]
+    fn call_Fx(function_pointer: *const u8, closure_data: *const u8, output: *mut u8) -> ();
+
+    #[link_name = "Main_rocMain_1_Fx_size"]
+    fn size_Fx() -> i64;
+}
+
+#[no_mangle]
+pub fn roc_fx_putChar(foo: i64) -> () {
+    let character = foo as u8 as char;
+    print!("{}", character);
+
+    ()
+}
+
+#[no_mangle]
+pub fn roc_fx_putLine(line: RocStr) -> () {
+    let bytes = line.as_slice();
+    let string = unsafe { std::str::from_utf8_unchecked(bytes) };
+    println!("{}", string);
+
+    ()
+}
+
+#[no_mangle]
+pub fn roc_fx_getLine() -> RocStr {
+    use std::io::{self, BufRead};
+
+    let stdin = io::stdin();
+    let line1 = stdin.lock().lines().next().unwrap().unwrap();
+
+    RocStr::from_slice_with_capacity(line1.as_bytes(), line1.len())
+}
+
+unsafe fn call_the_closure(function_pointer: *const u8, closure_data_ptr: *const u8) -> i64 {
+    let size = size_Fx() as usize;
+
+    alloca::with_stack_bytes(size, |buffer| {
+        let buffer: *mut std::ffi::c_void = buffer;
+        let buffer: *mut u8 = buffer as *mut u8;
+
+        call_Fx(
+            function_pointer,
+            closure_data_ptr as *const u8,
+            buffer as *mut u8,
+        );
+
+        let output = &*(buffer as *mut RocCallResult<i64>);
+
+        //        match output.into() {
+        //            Ok(v) => v,
+        //            Err(e) => panic!("failed with {}", e),
+        //        }
+        32
+    })
+}
+
+#[no_mangle]
+pub fn rust_main() -> isize {
+    println!("Running Roc closure");
+    let start_time = SystemTime::now();
+
+    let size = unsafe { roc_main_size() } as usize;
+    let layout = Layout::array::<u8>(size).unwrap();
+    let answer = unsafe {
+        let buffer = std::alloc::alloc(layout);
+
+        roc_main(buffer);
+
+        let output = &*(buffer as *mut RocCallResult<()>);
+
+        match output.into() {
+            Ok(()) => {
+                let function_pointer = {
+                    // this is a pointer to the location where the function pointer is stored
+                    // we pass just the function pointer
+                    let temp = buffer.offset(8) as *const i64;
+
+                    (*temp) as *const u8
+                };
+
+                let closure_data_ptr = buffer.offset(16);
+
+                call_the_closure(function_pointer as *const u8, closure_data_ptr as *const u8)
+            }
+            Err(msg) => {
+                std::alloc::dealloc(buffer, layout);
+
+                panic!("Roc failed with message: {}", msg);
+            }
+        }
+    };
+    let end_time = SystemTime::now();
+    let duration = end_time.duration_since(start_time).unwrap();
+
+    println!(
+        "Roc execution took {:.4} ms",
+        duration.as_secs_f64() * 1000.0,
+    );
+
+    // Exit code
+    0
+}