From ba5283694e7b0ea41b0eadc4beffa5f3a6f2ffc9 Mon Sep 17 00:00:00 2001
From: Brandon Dyck <brandon@dyck.us>
Date: Wed, 28 Jun 2023 16:26:08 -0600
Subject: [PATCH] Parse if-expressions

---
 ASTPrinter.cs   |  5 +++++
 Expr.g.cs       | 11 +++++++++++
 Parser.cs       | 16 ++++++++++++++++
 Program.cs      |  4 ++++
 ast_classes.fsx |  7 ++++++-
 5 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/ASTPrinter.cs b/ASTPrinter.cs
index a29998d..a331171 100644
--- a/ASTPrinter.cs
+++ b/ASTPrinter.cs
@@ -43,4 +43,9 @@ class ASTPrinter : IVisitor<string>
 	{
 		return parenthesize(expr.Op.lexeme, expr.Right);
 	}
+
+	public string visitIfExpr(If expr)
+	{
+		return parenthesize("if", expr.Condition, expr.Then, expr.Else);
+	}
 }
diff --git a/Expr.g.cs b/Expr.g.cs
index 290bbf4..1e0937d 100644
--- a/Expr.g.cs
+++ b/Expr.g.cs
@@ -14,6 +14,7 @@ public interface IVisitor<T> {
 	T visitGroupingExpr(Grouping expr);
 	T visitLiteralExpr(Literal expr);
 	T visitUnaryExpr(Unary expr);
+	T visitIfExpr(If expr);
 }
 public class Binary : Expr
 {
@@ -50,3 +51,13 @@ public class Unary : Expr
 		return visitor.visitUnaryExpr(this);
 	}
 }
+public class If : Expr
+{
+	public required Expr Condition { get; init; }
+	public required Expr Then { get; init; }
+	public required Expr Else { get; init; }
+	public override T accept<T>(IVisitor<T> visitor)
+	{
+		return visitor.visitIfExpr(this);
+	}
+}
diff --git a/Parser.cs b/Parser.cs
index 502e1ed..9db5c7b 100644
--- a/Parser.cs
+++ b/Parser.cs
@@ -143,6 +143,7 @@ class Parser
 	private Expr sum() => binaryLeft(product, TokenType.Minus, TokenType.Plus);
 
 	private Expr product() => binaryLeft(unary, TokenType.Slash, TokenType.Asterisk);
+
 	private Expr unary()
 	{
 		if (match(TokenType.Bang, TokenType.Minus))
@@ -151,6 +152,21 @@ class Parser
 			Expr right = unary();
 			return new Unary { Op = op, Right = right };
 		}
+		return ifExpr();
+	}
+
+	private Expr ifExpr()
+	{
+		if (match(TokenType.If))
+		{
+			Expr condition = expression();
+			consume(TokenType.Then, "Expect 'then' after condition.");
+			Expr thenCase = expression();
+			consume(TokenType.Else, "Expect 'else' after 'then' case.");
+			Expr elseCase = expression();
+			return new If { Condition = condition, Then = thenCase, Else = elseCase };
+		}
+
 		return primary();
 	}
 
diff --git a/Program.cs b/Program.cs
index e7acf23..efbc2dd 100644
--- a/Program.cs
+++ b/Program.cs
@@ -107,6 +107,7 @@ public enum TokenType
 	PipeLeft,
 	ComposeRight,
 	ComposeLeft,
+	If, Then, Else,
 	When,
 	Is,
 	Let,
@@ -136,6 +137,9 @@ class Scanner
 	private static readonly Dictionary<String, TokenType> keywords = new Dictionary<string, TokenType>()
 	{
 		{"def", TokenType.Def},
+		{"if", TokenType.If},
+		{"then", TokenType.Then},
+		{"else", TokenType.Else},
 		{"when", TokenType.When},
 		{"is", TokenType.Is},
 		{"let", TokenType.Let},
diff --git a/ast_classes.fsx b/ast_classes.fsx
index 5b7b09f..35ec3ed 100644
--- a/ast_classes.fsx
+++ b/ast_classes.fsx
@@ -14,7 +14,12 @@ let types =
           [ { Type = "System.Object"
               Name = "Value" } ] }
       { Name = "Unary"
-        Fields = [ { Type = "Token"; Name = "Op" }; { Type = "Expr"; Name = "Right" } ] } ]
+        Fields = [ { Type = "Token"; Name = "Op" }; { Type = "Expr"; Name = "Right" } ] }
+      { Name = "If"
+        Fields =
+          [ { Type = "Expr"; Name = "Condition" }
+            { Type = "Expr"; Name = "Then" }
+            { Type = "Expr"; Name = "Else" } ] } ]
 
 let visitorMethod baseName t = $"visit{t.Name}{baseName}"