haskell: classic program calculator.hs + nested constructor patterns (+5 tests, 402/402)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/haskell/tests/programs/calculator.hs

@@ -0,0 +1,40 @@
+-- calculator.hs — recursive descent expression evaluator.
+--
+-- Tokens are represented as an ADT; the parser threads a [Token] list
+-- through a custom Result type so pattern matching can destructure the
+-- pair (value, remaining-tokens) directly inside constructor patterns.
+--
+-- Operator precedence: * and / bind tighter than + and -.
+-- All operators are left-associative.
+
+data Token = TNum Int | TOp String
+data Result = R Int [Token]
+
+getV (R v _) = v
+getR (R _ r) = r
+
+eval ts = getV (parseExpr ts)
+
+parseExpr ts = parseExprRest (parseTerm ts)
+
+parseExprRest (R v (TOp "+":rest)) =
+  let t = parseTerm rest
+  in parseExprRest (R (v + getV t) (getR t))
+parseExprRest (R v (TOp "-":rest)) =
+  let t = parseTerm rest
+  in parseExprRest (R (v - getV t) (getR t))
+parseExprRest r = r
+
+parseTerm ts = parseTermRest (parseFactor ts)
+
+parseTermRest (R v (TOp "*":rest)) =
+  let t = parseFactor rest
+  in parseTermRest (R (v * getV t) (getR t))
+parseTermRest (R v (TOp "/":rest)) =
+  let t = parseFactor rest
+  in parseTermRest (R (v `div` getV t) (getR t))
+parseTermRest r = r
+
+parseFactor (TNum n:rest) = R n rest
+
+result = eval [TNum 2, TOp "+", TNum 3, TOp "*", TNum 4]