With the following code nhc98 prints 7, not 6 as I'd expect

Absolutely right, this is a bug. The defining feature that triggers the bug is that both equations for 'foo' use the same (n+k) pattern.

module Main where

data Wibble = Foo | Bar

main :: IO() main = putStrLn $ show $ foo 1 Bar

foo :: Int -> Wibble -> Int foo (n+1) Foo = 5 foo (n+1) Bar = 6 foo _ _ = 7

A patch is attached which fixes the problem. The patch is also available via the nhc98 download page. Regards, Malcolm Index: src/compiler98/Case.hs =================================================================== RCS file: /usr/src/master/nhc/src/compiler98/Case.hs,v retrieving revision 1.16 diff -u -r1.16 Case.hs --- src/compiler98/Case.hs 2001/05/03 17:17:03 1.16 +++ src/compiler98/Case.hs 2001/09/14 16:00:12 @@ -370,7 +370,8 @@ caseTranslate v (concatMap (getTrans.fst) x) >=> mapS (matchNK v ces) x >>>= \ nks -> def >>>= \ e2 -> - optFatBar (f (foldr ($) PosExpFail nks)) e2 +--optFatBar (f (foldr ($) PosExpFail nks)) e2 + optFatBar (f (foldr1 (PosExpFatBar True) nks)) e2 matchOne (ce:ces) (PatternIf x) def = varExp ce >>>= \ (v,f,ce) -> @@ -475,7 +476,8 @@ match ces funs (unitS PosExpFail) >>>= \ exp -> unitS (PosAltInt noPos i exp) -matchNK :: Int -> [PosExp] -> (ExpI,Fun Int) -> CaseFun (PosExp->PosExp) +--matchNK :: Int -> [PosExp] -> (ExpI,Fun Int) -> CaseFun (PosExp->PosExp) +matchNK :: Int -> [PosExp] -> (ExpI,Fun Int) -> CaseFun PosExp matchNK v ces (PatNplusK pos n n' k kle ksub, fun) = match ces [fun] (unitS PosExpFail) >>>= \ exp -> caseDecl @@ -485,7 +487,8 @@ caseDecl (DeclFun pos n [Fun [] (Unguarded ksub) (DeclsScc [])]) >>>= \ binding -> unitS - (\f-> PosExpLet pos local (PosExpIf pos cond (PosExpLet pos binding exp) f)) +-- (\f-> PosExpLet pos local (PosExpIf pos cond (PosExpLet pos binding exp) f)) + (PosExpLet pos local (PosExpIf pos cond (PosExpLet pos binding exp) PosExpFail)) ------------------