From 2e6172c115e5b0dd8694d782bdc1b4988c23fbc1 Mon Sep 17 00:00:00 2001
From: pingu <nor@acorneroftheweb.com>
Date: Mon, 17 Nov 2025 17:02:18 +0100
Subject: [PATCH] Tehee

---
 3/Assignment.hs | 25 +++++++++++++++++++++++++
 3/assig.org     | 30 +++++++++++++++++++++++-------
 3/chi.cabal     |  5 +++++
 3 files changed, 53 insertions(+), 7 deletions(-)
 create mode 100644 3/Assignment.hs

diff --git a/3/Assignment.hs b/3/Assignment.hs
new file mode 100644
index 0000000..de206ff
--- /dev/null
+++ b/3/Assignment.hs
@@ -0,0 +1,25 @@
+{-# Language LambdaCase #-}
+module Assignment where
+
+import Chi
+
+subst :: Variable -> Exp -> Exp -> Exp
+subst var e = \case
+  Apply e1 e2 -> Apply (subst var e e1) (subst var e e2)
+  Lambda x e' -> Lambda x $ if var /= x then (subst var e e') else e'
+  Var x -> if var == x then e else Var x
+  Const c es -> Const c $ map (subst var e) es
+  Rec x e' -> Rec x $ if var /= x then (subst var e e') else e' 
+  Case e' branches -> Case (subst var e e') $ map substBr branches
+    where
+      substBr :: Br -> Br
+      substBr (Branch c vs e') = Branch c vs $ if var `notElem` vs then (subst var e e') else e'
+
+eval' :: Exp -> Reader (Map Variable Exp) Exp
+eval' = undefined
+
+eval :: Exp -> Exp
+eval = undefined
+
+main :: IO ()
+main = getLine >>= print . eval . parse
diff --git a/3/assig.org b/3/assig.org
index bf04add..ebe45c2 100644
--- a/3/assig.org
+++ b/3/assig.org
@@ -3,10 +3,8 @@
 #+latex_header: \usepackage{parskip}
 
 * Supporting files
-
-~This~ time you can make use of a [[http://bnfc.digitalgrammars.com/][BNFC]] specification ([[https://chalmers.instructure.com/courses/36941/file_contents/course%20files/chi/Chi.cf][~Chi.cf~]]) of a variant of the concrete syntax of $\chi$. This grammar gives specific rules for variable and constructor names, and differs from the concrete syntax presented in the lectures in some ways:
-# TODO: Fix '
-- Variables and constructors are sequences of letters (~a~ to ~z~ and ~A~ to ~Z~), digits (~0~ to ~9~), underscores (~_~), dashes (~-~) and primes ('), and variables have to start with lower-case letters or underscores, whereas constructors have to start with upper-case letters.
+This time you can make use of a [[http://bnfc.digitalgrammars.com/][BNFC]] specification ([[https://chalmers.instructure.com/courses/36941/file_contents/course%20files/chi/Chi.cf][~Chi.cf~]]) of a variant of the concrete syntax of $\chi$. This grammar gives specific rules for variable and constructor names, and differs from the concrete syntax presented in the lectures in some ways:
+- Variables and constructors are sequences of letters (~a~ to ~z~ and ~A~ to ~Z~), digits (~0~ to ~9~), underscores (~_~), dashes (~-~) and primes (='=), and variables have to start with lower-case letters or underscores, whereas constructors have to start with upper-case letters.
 - Lambdas are written using backslash characters (~\~) and arrows are written using the string ~->~.
 - End-of-line comments of the form ~-- ...~ are allowed, as are (non-nested) comments of the form ~{- ... -}~.
 - There can be a final comma after a non-empty list of constructor arguments, and there can be a final semicolon after a non-empty list of branches.
@@ -61,10 +59,14 @@ Consider the $\chi$ term /t/ with concrete syntax $C (\lambda z.z)$:
 
 In the abstract syntax the constructor ~C~ should be represented by $\underline{C} \in \text{Const}$, and the variable ~z~ by $\underline{z} \in \text{Var}$. When giving the $\chi$ representation of the term you should use (the representation of) the number 0 to represent $\underline{C}$, and 1 to represent $\underline{z}$.
 *** Answer
-The abstract syntax is the following: $\text{const}\ \underline{C}\  (\text{cons}\ (\text{lambda}\ \underline{z}\ (\text{var}\ \underline{z}))\ \text{nil})$.
+The abstract syntax is the following:
+\[ \text{const}\ \underline{C}\  (\text{cons}\ (\text{lambda}\ \underline{z}\ (\text{var}\ \underline{z}))\ \text{nil}) \]
 
-The concrete syntax of the standard representation is the following: $Const(\ulcorner \underline{C} \urcorner, Cons(Lambda(\ulcorner \underline{z} \urcorner, Cons (Var(\ulcorner \underline{z} \urcorner), Nil())), Nil())$
-# TODO: check parenthesis above
+The concrete syntax of the standard representation is the following:
+\[ \text{Const}(\ulcorner \underline{C} \urcorner, \text{Cons}(\text{Lambda}(\ulcorner \underline{z} \urcorner, \text{Cons} (\text{Var}(\ulcorner \underline{z} \urcorner), \text{Nil}())), \text{Nil}())) \]
+
+The abstract syntax of the standard representation, given that $\underline{z}$ and $\underline{C}$ corresponds to zero:
+\[ \text{Const}(\text{Zero}(), \text{Cons}(\text{Lambda}(\text{Zero}(), \text{Cons} (\text{Var}(\text{Zero}()), \text{Nil}())), \text{Nil}())) \]
 
 ** (2p) [BN]
 
@@ -82,6 +84,7 @@ Test your implementation. Here are some test cases that must work:
 | ~z~        | $C(\lambda z . z)$     | $\text{case}\ z\ \text{of}\ \{ C(z) \rightarrow z \}$ | $\text{case}\ C(\lambda z. z) \ \text{of}\ \{ C(z) \rightarrow z \}$ |
 
 *** Answer
+See Assignment.hs
 
 ** (1p)
 Implement multiplication of natural numbers in $\chi$, using the representation of natural numbers given in the $\chi$ specification.
@@ -89,6 +92,19 @@ Implement multiplication of natural numbers in $\chi$, using the representation
 Hint: If you want to make use of addition in the implementation of multiplication, then you can define multiplication using a free variable ~add~, and use the substitution operation from the previous exercise to substitute a complete (closed) implementation of addition for this variable.
 
 *** Answer
+#+begin_src chi
+\m. rec mult = \n. case n of
+  { Zero() -> Zero()
+  ; Succ(n) -> add m (mult n)
+  } 
+#+end_src
+where we substitute ~add~ for the following:
+#+begin_src chi
+\m. rec add = \n. case n of
+  { Zero() -> m
+  ; Succ(n) -> Suc(add n)
+  } 
+#+end_src
 
 ** (2p) [BN]
 Implement an interpreter for $\chi$. The interpreter should be a partial function from closed terms to values, and should implement the operational semantics of $\chi$.
diff --git a/3/chi.cabal b/3/chi.cabal
index cfd37a4..7ee5807 100644
--- a/3/chi.cabal
+++ b/3/chi.cabal
@@ -26,3 +26,8 @@ library
     PrintChi
   hs-source-dirs:
     .
+
+executable interpreter
+    main-is: Assignment.hs
+    build-depends: base
+                 , chi