feat: Add format! macro

Author: bch29

src/format_macro.rs

@@ -0,0 +1,197 @@
+//! Implementation of the `format!` macro
+use gluon_codegen::Trace;
+
+use crate::{
+    base::{
+        ast::{self, AstClone, SpannedExpr},
+        pos,
+        symbol::{Symbol, Symbols},
+        types::TypeCache,
+    },
+    parser::parse_expr,
+    vm::macros::{self, Macro, MacroExpander, MacroFuture},
+};
+
+/**
+ * Format macro with expressions embedded in the format string
+ *
+ * ```
+ * let x = 1
+ * format! "x is {x}, x + 1 is {x+1}"
+ * ```
+ */
+#[derive(Trace)]
+#[gluon(crate_name = "vm")]
+pub struct Format;
+
+impl Macro for Format {
+    fn expand<'r, 'a: 'r, 'b: 'r, 'c: 'r, 'ast: 'r>(
+        &self,
+        _env: &'b mut MacroExpander<'a>,
+        symbols: &'c mut Symbols,
+        arena: &'b mut ast::OwnedArena<'ast, Symbol>,
+        args: &'b mut [SpannedExpr<'ast, Symbol>],
+    ) -> MacroFuture<'r, 'ast> {
+        Box::pin(async move {
+            let arg = match args {
+                [arg] => (arg),
+                _ => {
+                    return Err(macros::Error::message(format!(
+                        "format! expects 1 argument"
+                    )))
+                }
+            };
+
+            let expr: &ast::Expr<Symbol> = &arg.value;
+
+            let format_string = match expr {
+                ast::Expr::Literal(ast::Literal::String(text)) => text,
+                _ => {
+                    return Err(macros::Error::message(format!(
+                        "format! expects a string argument"
+                    )))
+                }
+            };
+
+            let span = arg.span;
+
+            let sp = |e: ast::Expr<'ast, Symbol>| pos::spanned(span, e);
+
+            let show_ident = ast::TypedIdent::new(symbols.simple_symbol("show"));
+            let show_func = arena.alloc(sp(ast::Expr::Ident(show_ident)));
+
+            let append_ident = ast::TypedIdent::new(symbols.simple_symbol("(++)"));
+            let append_func = arena.alloc(sp(ast::Expr::Ident(append_ident)));
+
+            let show_expr = |e: ast::SpannedExpr<'ast, Symbol>| {
+                let func = show_func.ast_clone(arena.borrow());
+
+                sp(ast::Expr::App {
+                    func,
+                    implicit_args: arena.alloc_extend(vec![]),
+                    args: arena.alloc_extend(vec![e]),
+                })
+            };
+
+            let app_exprs = |lhs, rhs| {
+                let func = append_func.ast_clone(arena.borrow());
+
+                sp(ast::Expr::App {
+                    func,
+                    implicit_args: arena.alloc_extend(vec![]),
+                    args: arena.alloc_extend(vec![lhs, rhs]),
+                })
+            };
+
+            let literal_expr = |val: String| sp(ast::Expr::Literal(ast::Literal::String(val)));
+
+            let type_cache = TypeCache::new();
+
+            let mut remaining = format_string.as_str();
+
+            let mut result_expr = None;
+
+            while let Some(find_result) = find_expr(remaining)? {
+                remaining = find_result.remaining;
+
+                let sub_expr =
+                    parse_expr(arena.borrow(), symbols, &type_cache, find_result.expr)
+                        .map_err(|err| {
+                            macros::Error::message(format!(
+                                "format! could not parse subexpression: {}",
+                                err
+                            ))
+                        })?;
+
+                let part_expr = app_exprs(
+                    literal_expr(find_result.prefix.to_owned()),
+                    show_expr(sub_expr),
+                );
+
+                result_expr = match result_expr.take() {
+                    None => Some(part_expr),
+                    Some(prev_expr) => Some(app_exprs(prev_expr, part_expr)),
+                };
+            }
+
+            let result_expr = match result_expr.take() {
+                None => literal_expr(remaining.to_owned()),
+                Some(result_expr) => {
+                    if remaining.is_empty() {
+                        result_expr
+                    } else {
+                        app_exprs(result_expr, literal_expr(remaining.to_owned()))
+                    }
+                }
+            };
+
+            Ok(result_expr.into())
+        })
+    }
+}
+
+const OPEN_BRACE: &'static str = "{";
+const CLOSE_BRACE: &'static str = "}";
+
+struct FindExprResult<'a> {
+    prefix: &'a str,
+    expr: &'a str,
+    remaining: &'a str,
+}
+
+fn find_expr<'a>(format_str: &'a str) -> Result<Option<FindExprResult<'a>>, macros::Error> {
+    let prefix_end_ix = match format_str.find(OPEN_BRACE) {
+        None => return Ok(None),
+        Some(ix) => ix,
+    };
+
+    let prefix = &format_str[..prefix_end_ix];
+    let expr_start = OPEN_BRACE.len() + prefix_end_ix;
+
+    let mut brace_depth = 1;
+
+    let mut expr_end = expr_start;
+    let mut brace_end = expr_start;
+
+    while brace_depth != 0 {
+        let next_open = format_str[brace_end..].find(OPEN_BRACE);
+        let next_close = format_str[brace_end..].find(CLOSE_BRACE);
+
+        let (brace_ix, is_close) = match (next_open, next_close) {
+            (None, None) => break,
+            (None, Some(ix)) => (ix, true),
+            (Some(ix), None) => (ix, false),
+            (Some(open_ix), Some(close_ix)) => {
+                if open_ix < close_ix {
+                    (open_ix, false)
+                } else {
+                    (close_ix, true)
+                }
+            }
+        };
+
+        expr_end = brace_end + brace_ix;
+        brace_end = if is_close {
+            brace_depth -= 1;
+            expr_end + OPEN_BRACE.len()
+        } else {
+            brace_depth += 1;
+            expr_end + CLOSE_BRACE.len()
+        };
+    }
+
+    if brace_depth != 0 {
+        return Err(macros::Error::message(format!("mismatched braces")));
+    }
+
+    let expr = &format_str[expr_start..expr_end];
+
+    let remaining = &format_str[brace_end..];
+
+    Ok(Some(FindExprResult {
+        prefix,
+        expr,
+        remaining,
+    }))
+}
+

src/lib.rs

@@ -49,6 +49,7 @@ pub mod compiler_pipeline;
 #[macro_use]
 pub mod import;
 pub mod lift_io;
+pub mod format_macro;
 #[doc(hidden)]
 pub mod query;
 pub mod std_lib;
@@ -958,6 +959,8 @@ impl VmBuilder {
             }
 
             macros.insert(String::from("lift_io"), lift_io::LiftIo);
+
+            macros.insert(String::from("format"), format_macro::Format);
         }
 
         add_extern_module_with_deps(