From 48e2adc69963f090bcfbfbf7dd89fd2c1b849d45 Mon Sep 17 00:00:00 2001
From: skymanbp <57272723+skymanbp@users.noreply.github.com>
Date: Thu, 24 Sep 2026 22:33:14 -0400
Subject: [PATCH] fix: pow and daysBetween are not commutative

---
 .../src/DataFrame/Functions.hs                |  4 +--
 docs/base/haskell_for_data_analysis.md        |  2 +-
 docs/haskell_for_data_analysis.md             |  2 +-
 tests/Functions.hs                            | 33 +++++++++++++++++++
 4 files changed, 37 insertions(+), 4 deletions(-)

diff --git a/dataframe-operations/src/DataFrame/Functions.hs b/dataframe-operations/src/DataFrame/Functions.hs
index e9f0803e..09b52318 100644
--- a/dataframe-operations/src/DataFrame/Functions.hs
+++ b/dataframe-operations/src/DataFrame/Functions.hs
@@ -460,7 +460,7 @@ zScore :: Expr Double -> Expr Double
 zScore c = (c - mean c) / stddev c
 
 pow :: (Columnable a, Num a) => Expr a -> Int -> Expr a
-pow expr i = lift2Decorated (^) "pow" (Just "^") True 8 expr (Lit i)
+pow expr i = lift2Decorated (^) "pow" (Just "^") False 8 expr (Lit i)
 {-# SPECIALIZE pow :: Expr Double -> Int -> Expr Double #-}
 {-# SPECIALIZE pow :: Expr Float -> Int -> Expr Float #-}
 {-# SPECIALIZE pow :: Expr Int -> Int -> Expr Int #-}
@@ -645,7 +645,7 @@ daysBetween =
         (\d1 d2 -> fromIntegral (diffDays d1 d2))
         "daysBetween"
         Nothing
-        True
+        False
         2
 
 bind ::
diff --git a/docs/base/haskell_for_data_analysis.md b/docs/base/haskell_for_data_analysis.md
index 6e463905..987f8462 100644
--- a/docs/base/haskell_for_data_analysis.md
+++ b/docs/base/haskell_for_data_analysis.md
@@ -661,7 +661,7 @@ TIO.putStrLn $ D.toMarkdown withDates
 
 ```haskell
 -- This requires two Day columns; here we illustrate the pattern:
--- D.derive "days_until" (F.daysBetween today_col deadline_col) df
+-- D.derive "days_until" (F.daysBetween deadline_col today_col) df
 ```
 
 ### Opting into stronger type safety
diff --git a/docs/haskell_for_data_analysis.md b/docs/haskell_for_data_analysis.md
index 6f6f180d..fc9a66ff 100644
--- a/docs/haskell_for_data_analysis.md
+++ b/docs/haskell_for_data_analysis.md
@@ -1039,7 +1039,7 @@ TIO.putStrLn $ D.toMarkdown withDates
 
 ```haskell
 -- This requires two Day columns; here we illustrate the pattern:
--- D.derive "days_until" (F.daysBetween today_col deadline_col) df
+-- D.derive "days_until" (F.daysBetween deadline_col today_col) df
 ```
 
 
diff --git a/tests/Functions.hs b/tests/Functions.hs
index 24457065..a225bee5 100644
--- a/tests/Functions.hs
+++ b/tests/Functions.hs
@@ -3,6 +3,7 @@
 
 module Functions where
 
+import Data.Time.Calendar (Day, fromGregorian)
 import qualified DataFrame as D
 import DataFrame.Functions (
     sanitize,
@@ -79,8 +80,40 @@ testSum =
             (D.derive "sum" (F.sum (F.col @Int "A")) df)
         )
 
+testPow :: Test
+testPow =
+    TestCase
+        ( assertEqual
+            "pow of a compound base"
+            [4, 9, 16, 25, 36, 49, 64, 81, 100, 121]
+            ( D.columnAsList @Int
+                (F.col "sq")
+                (D.derive "sq" (F.pow (F.col @Int "A" + F.lit 1) 2) df)
+            )
+        )
+
+testDaysBetween :: Test
+testDaysBetween =
+    TestCase
+        ( assertEqual
+            "daysBetween d1 d2 is d1 minus d2 in either argument order"
+            ([-9], [9])
+            ( days (F.col @Day "start") (F.col @Day "end")
+            , days (F.col @Day "end") (F.col @Day "start")
+            )
+        )
+  where
+    dates =
+        D.fromNamedColumns
+            [ ("start", DI.fromList [fromGregorian 2024 3 1])
+            , ("end", DI.fromList [fromGregorian 2024 3 10])
+            ]
+    days a b = D.columnAsList @Int (F.col "d") (D.derive "d" (F.daysBetween a b) dates)
+
 tests :: [Test]
 tests =
     [ TestLabel "sanitizeIdentifiers" sanitizeIdentifiers
     , TestLabel "testSum" testSum
+    , TestLabel "testPow" testPow
+    , TestLabel "testDaysBetween" testDaysBetween
     ]