From b3909efb3a6bf76870b686b5062f9a4282fbdd66 Mon Sep 17 00:00:00 2001
From: Mathias Magnusson <mathias@magnusson.space>
Date: Thu, 24 Jul 2025 20:38:56 +0200
Subject: `let x = 1` -> `x := 1`

---
 fibonacci.hgn |  9 +++++----
 src/Lexer.zig |  6 ++++--
 src/parse.zig | 35 +++++++++++++++++++----------------
 3 files changed, 28 insertions(+), 22 deletions(-)

diff --git a/fibonacci.hgn b/fibonacci.hgn
index 00c0638..01d9333 100644
--- a/fibonacci.hgn
+++ b/fibonacci.hgn
@@ -1,8 +1,9 @@
-let a = 0
-let b = 1
-let n = read_int(0)
+a := 0
+b := 1
+n := 10
+# n := read_int(0)
 while n > 0 {
-    let c = a + b
+    c := a + b
     a = b
     b = c
     n = n - 1
diff --git a/src/Lexer.zig b/src/Lexer.zig
index 4665379..4b664fe 100644
--- a/src/Lexer.zig
+++ b/src/Lexer.zig
@@ -13,6 +13,7 @@ pub const Token = struct {
         plus,
         minus,
         equal,
+        colon,
         invalid,
         eof,
         identifier,
@@ -22,7 +23,6 @@ pub const Token = struct {
         right_angle_equal,
 
         // Keywords
-        let,
         @"if",
         @"else",
         @"while",
@@ -34,6 +34,7 @@ pub const Location = struct {
     end: usize,
 
     pub fn combine(a: Location, b: Location) Location {
+        if (a.start == b.start and a.end == b.end) return a;
         std.debug.assert(a.end <= b.start);
         return .{ .start = @min(a.start, b.start), .end = @max(a.end, b.end) };
     }
@@ -83,6 +84,7 @@ fn getNext(self: *Self) Token {
         '+' => self.create(.plus),
         '-' => self.create(.minus),
         '=' => self.create(.equal),
+        ':' => self.create(.colon),
         '<' => if (self.eatIfEqual('='))
             self.create(.left_angle_equal)
         else
@@ -135,7 +137,7 @@ fn identifierOrKeyword(self: *Self) Token {
         }
         const value = self.source[self.start..self.pos];
         return self.create(switch (std.meta.stringToEnum(Token.Type, value) orelse .invalid) {
-            .let, .@"if", .@"else", .@"while" => |t| t,
+            .@"if", .@"else", .@"while" => |t| t,
             else => .identifier,
         });
     }
diff --git a/src/parse.zig b/src/parse.zig
index 26f32e9..bd6b000 100644
--- a/src/parse.zig
+++ b/src/parse.zig
@@ -64,9 +64,9 @@ pub const Stmt = struct {
         return switch (self.type) {
             .expr => |expr| writer.print("{}", .{fmt(expr, source, indent)}),
             .block => |b| writer.print("{}", .{fmt(b, source, indent)}),
-            .assign_var => |assign_var| writer.print("{s}{s} = {}", .{
-                if (assign_var.is_decl) "let " else "",
+            .assign_var => |assign_var| writer.print("{s} {s} {}", .{
                 assign_var.ident.getIdent(source),
+                if (assign_var.is_decl) ":=" else "=",
                 fmt(assign_var.value, source, indent),
             }),
             .@"while" => |@"while"| try writer.print("while {} {}", .{
@@ -149,7 +149,13 @@ pub const Expr = struct {
     }
 };
 
-const ParseError = error{ OutOfMemory, ExpectedRightParen, UnexpectedToken, InvalidAssignTarget };
+const ParseError = error{
+    OutOfMemory,
+    ExpectedRightParen,
+    UnexpectedToken,
+    InvalidAssignTarget,
+    ExprStatementMustBeCall,
+};
 
 pub fn file(allocator: Allocator, lexer: *Lexer) !Block {
     var stmts: std.ArrayList(Stmt) = .init(allocator);
@@ -177,16 +183,6 @@ fn block(allocator: Allocator, lexer: *Lexer) !Block {
 
 fn statement(allocator: Allocator, lexer: *Lexer) ParseError!Stmt {
     switch (lexer.peek().type) {
-        .let => {
-            const let = lexer.next();
-            const ident = try mustEat(lexer, .identifier);
-            _ = try mustEat(lexer, .equal);
-            const value = try expression(allocator, lexer);
-            return .{
-                .loc = let.loc.combine(value.loc),
-                .type = .{ .assign_var = .{ .ident = ident.loc, .is_decl = true, .value = value } },
-            };
-        },
         .left_curly => {
             const b = try block(allocator, lexer);
             return .{
@@ -208,7 +204,12 @@ fn statement(allocator: Allocator, lexer: *Lexer) ParseError!Stmt {
         },
         else => {
             const lhs = try expression(allocator, lexer);
-            if (lexer.peek().type == .equal) {
+
+            const colon = if (lexer.peek().type == .colon)
+                try mustEat(lexer, .colon)
+            else
+                null;
+            if (colon != null or lexer.peek().type == .equal) {
                 _ = try mustEat(lexer, .equal);
                 const value = try expression(allocator, lexer);
                 if (lhs.type != .identifier) {
@@ -216,9 +217,11 @@ fn statement(allocator: Allocator, lexer: *Lexer) ParseError!Stmt {
                     return error.InvalidAssignTarget;
                 }
                 return .{
-                    .loc = lhs.loc,
-                    .type = .{ .assign_var = .{ .ident = lhs.loc, .is_decl = false, .value = value } },
+                    .loc = lhs.loc.combine(value.loc),
+                    .type = .{ .assign_var = .{ .ident = lhs.loc, .is_decl = colon != null, .value = value } },
                 };
+            } else if (lhs.type != .call) {
+                return error.ExprStatementMustBeCall;
             }
             return .{
                 .loc = lhs.loc,
-- 
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