From 84442e4e3a53b5c6140ce44a8ddad12541894509 Mon Sep 17 00:00:00 2001
From: Adrian Cochrane <adrian@openwork.nz>
Date: Tue, 11 Apr 2023 09:23:31 +1200
Subject: [PATCH] Reference-document (unused) calc() implementation.

---
 Graphics/Layout/Arithmetic.hs | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/Graphics/Layout/Arithmetic.hs b/Graphics/Layout/Arithmetic.hs
index 50c5f45..3ee01da 100644
--- a/Graphics/Layout/Arithmetic.hs
+++ b/Graphics/Layout/Arithmetic.hs
@@ -7,8 +7,11 @@ import Data.Scientific (toRealFloat)
 import GHC.Real (infinity)
 import Data.Text (unpack, Text)
 import qualified Data.Text as Txt
+import Debug.Trace (trace) -- For error reporting.
 
+-- | Parsed calc() expression. As a postfix arithmatic expression.
 data Opcode n = Seq | Add | Subtract | Multiply | Divide | Func Text | Num n deriving Show
+-- | Parse a calc() expression.
 parseCalc :: [Token] -> [Opcode (Float, String)] -> [Opcode (Float, String)]
 parseCalc (Number _ n:toks) stack = Num (val2float n, ""):parseCalc toks stack
 parseCalc (Percentage _ n:toks) stack = Num (val2float n, "%"):parseCalc toks stack
@@ -45,6 +48,7 @@ parseCalc [] [] = []
 parseCalc [] stack = parseCalc [RightParen] stack
 parseCalc _ _ = [Func "invalid"]
 
+-- | Parse an operator char.
 op :: Char -> Opcode n
 op '+' = Add
 op '-' = Subtract
@@ -55,6 +59,7 @@ op _ = Func "invalid"
 
 -- Do operands counts line up? Are we dividing by 0?
 -- Also I see concerns about whether units line up. Not bothering verifying that.
+-- | Verify that a parsed math expression can be properly evaluated.
 verifyCalc :: [Opcode (Float, String)] -> [Bool] -> Bool
 verifyCalc (Seq:expr) stack = verifyCalc expr stack
 verifyCalc (Add:expr) (_:_:stack) = verifyCalc expr (True:stack)
@@ -72,6 +77,7 @@ verifyCalc (Func "clamp":expr) (_:_:_:stack) = verifyCalc expr (True:stack)
 verifyCalc [] [_] = True
 verifyCalc _ _ = False
 
+-- | Evaluate a parsed calc() expression.
 evalCalc :: [Opcode Float] -> [Float] -> Float
 evalCalc (Seq:expr) stack = evalCalc expr stack -- The function args off
 evalCalc (Add:expr) (y:x:stack) = evalCalc expr ((x + y):stack)
@@ -103,8 +109,15 @@ evalCalc (Func "sqrt":expr) (x:stack) = evalCalc expr (sqrt x:stack)
 evalCalc (Func "tan":expr) (x:stack) = evalCalc expr (tan x:stack)
 
 evalCalc [] [ret] = ret
-evalCalc _ _ = error "Verification should have caught this error!"
+evalCalc [] stack@(ret:_) =
+    trace ("Verification should have caught this error! " ++ show stack) ret
+evalCalc [] [] = trace "Verification should have caught this error! Stack underflow!" 0
+evalCalc (op:_) (ret:_) =
+    trace ("Verification should have caught this error! Unsupported op " ++ show op) ret
+evalCalc (op:_) [] =
+    trace ("Verification should have caught this error! Unsupported op " ++ show op) 0
 
+-- | Convert all numbers in an expression via the given callback.
 mapCalc :: (a -> b) -> [Opcode a] -> [Opcode b]
 mapCalc cb (Num x:toks) = Num (cb x):mapCalc cb toks
 -- GHC demanded more verbosity...
@@ -116,9 +129,11 @@ mapCalc cb (Divide:toks) = Divide:mapCalc cb toks
 mapCalc cb (Func f':toks) = Func f':mapCalc cb toks
 mapCalc _ [] = []
 
+-- | Convert from a tokenized NumericValue to a Float.
 val2float :: NumericValue -> Float
 val2float (NVInteger n) = fromIntegral n
 val2float (NVNumber n) = toRealFloat n
 
+-- | Convert from a rational value to a float.
 f :: Rational -> Float
 f = fromRational
-- 
2.30.2