From 6c26bcb5eb0c3736aae13d9411610abb6fcd81a9 Mon Sep 17 00:00:00 2001
From: Smaug123 <patrick+github@patrickstevens.co.uk>
Date: Mon, 22 Jan 2024 19:22:36 +0000
Subject: [PATCH] If-then-else

---
 PrattParser.Test/TestParser.fs | 13 ++++++++++++-
 PrattParser/Domain.fs          |  3 +++
 PrattParser/Parser.fs          | 18 ++++++++++++++++++
 3 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/PrattParser.Test/TestParser.fs b/PrattParser.Test/TestParser.fs
index ea8521c..ae9eee1 100644
--- a/PrattParser.Test/TestParser.fs
+++ b/PrattParser.Test/TestParser.fs
@@ -16,7 +16,18 @@ module TestParser =
             "-a!", Expr.unaryMinus (Expr.factorial (Expr.var "a"))
             "-a! + b", Expr.plus (Expr.unaryMinus (Expr.factorial (Expr.var "a"))) (Expr.var "b")
             "(-a)! + b", Expr.plus (Expr.factorial (Expr.paren (Expr.unaryMinus (Expr.var "a")))) (Expr.var "b")
-            // TODO: if-then-else
+            "if x then y else z", Expr.ifThenElse (Expr.var "x") (Expr.var "y") (Expr.var "z")
+            "if x then y", Expr.ifThen (Expr.var "x") (Expr.var "y")
+            "1 + if x then y", Expr.plus (Expr.constInt 1) (Expr.ifThen (Expr.var "x") (Expr.var "y"))
+            "if x + 1 then y else z + 3",
+            Expr.ifThenElse
+                (Expr.plus (Expr.var "x") (Expr.constInt 1))
+                (Expr.var "y")
+                (Expr.plus (Expr.var "z") (Expr.constInt 3))
+            "(if x + 1 then y else z) + 3",
+            Expr.plus
+                (Expr.paren (Expr.ifThenElse (Expr.plus (Expr.var "x") (Expr.constInt 1)) (Expr.var "y") (Expr.var "z")))
+                (Expr.constInt 3)
 
             "g x y + a * (func b c)",
             let gXY =
diff --git a/PrattParser/Domain.fs b/PrattParser/Domain.fs
index 8c952e0..bf99ab3 100644
--- a/PrattParser/Domain.fs
+++ b/PrattParser/Domain.fs
@@ -12,6 +12,7 @@ type Expr =
     | Factorial of Expr
     | Paren of Expr
     | IfThenElse of Expr * Expr * Expr
+    | IfThen of Expr * Expr
 
 [<RequireQualifiedAccess>]
 module Expr =
@@ -28,6 +29,8 @@ module Expr =
     let ifThenElse ifClause thenClause elseClause =
         Expr.IfThenElse (ifClause, thenClause, elseClause)
 
+    let ifThen ifClause thenClause = Expr.IfThen (ifClause, thenClause)
+
 [<RequireQualifiedAccess>]
 type TokenType =
     | Plus
diff --git a/PrattParser/Parser.fs b/PrattParser/Parser.fs
index 6a90d38..9089bb5 100644
--- a/PrattParser/Parser.fs
+++ b/PrattParser/Parser.fs
@@ -71,6 +71,24 @@ module Parser =
 
                 Expr.paren contents, rest
 
+            elif firstToken.Type = TokenType.If then
+                let ifClause, rest = parseInner inputString rest 0
+
+                match rest with
+                | [] -> failwith "if requires a trailing then"
+                | head :: _ when head.Type <> TokenType.Then -> failwithf "if was not followed by then, got: %+A" head
+                | _ :: rest ->
+
+                let thenClause, rest = parseInner inputString rest 0
+
+                match rest with
+                | [] -> Expr.ifThen ifClause thenClause, rest
+                | head :: _ when head.Type <> TokenType.Else -> Expr.ifThen ifClause thenClause, rest
+                | _ :: rest ->
+
+                let elseClause, rest = parseInner inputString rest 0
+                Expr.ifThenElse ifClause thenClause elseClause, rest
+
             else
 
             match Token.prefixPrecedence firstToken.Type with