Hi, I have two alternatives to specify a specific function. They both compile ok, but the first one crashes (Stack space overflow) while the second one runs fine. I use GHC 6.10.1 on windowsXP Alternative 1: antecedent :: Rule -> Expression antecedent r = case r of Ru{} -> if (rrsrt r == AlwaysExpr) then error ("(Module ADLdataDef:) illegal call to antecedent of rule "++show r) else rrant r Sg{} -> antecedent (srsig r) Gc{} -> Tm (grspe r) Fr{} -> frcmp r Alternative 2: antecedent :: Rule -> Expression antecedent r@(Ru AlwaysExpr _ _ _ _ _ _ _ _) = error ("(Module ADLdef:) illegal call to antecedent of rule "++show r) antecedent (Ru _ a _ _ _ _ _ _ _) = a antecedent (Sg _ rule _ _ _ _ _) = antecedent rule antecedent (Gc _ d _ _ _ _ _) = Tm d antecedent (Fr _ _ e _) = e Both alternatives compile, but if i use Alternative 2, then my program runs fine. If I use Alternative 1 instead, I get a stack space overflow. I would think that both alternatives would have the same semantics. So i am surprised that one runs fine, while the other one crashes. Could anyone explain what is going on? Thanks! Han Joosten ---------------------------- Might help, here is the data definition: data Rule = -- Ru c antc p cons cpu expla sgn nr pn Ru { rrsrt :: RuleType -- ^ One of the following: -- | Implication if this is an implication; -- | Equivalence if this is an equivalence; -- | AlwaysExpr if this is an ALWAYS expression. , rrant :: Expression -- ^ Antecedent , rrfps :: FilePos -- ^ Position in the ADL file , rrcon :: Expression -- ^ Consequent , r_cpu :: Expressions -- ^ This is a list of subexpressions, which must be computed. , rrxpl :: String -- ^ Explanation , rrtyp :: (Concept,Concept) -- ^ Sign of this rule , runum :: Int -- ^ Rule number , r_pat :: String -- ^ Name of pattern in which it was defined. } -- Sg p rule expla sgn nr pn signal | Sg { srfps :: FilePos -- ^ position in the ADL file , srsig :: Rule -- ^ the rule to be signalled , srxpl :: String -- ^ explanation , srtyp :: (Concept,Concept) -- ^ type , runum :: Int -- ^ rule number , r_pat :: String -- ^ name of pattern in which it was defined. , srrel :: Declaration -- ^ the signal relation } -- Gc p antc cons cpu _ _ _ | Gc { grfps :: FilePos -- ^ position in the ADL file , grspe :: Morphism -- ^ specific , grgen :: Expression -- ^ generic , r_cpu :: Expressions -- ^ This is a list of subexpressions, which must be computed. , grtyp :: (Concept,Concept) -- ^ declaration , runum :: Int -- ^ rule number , r_pat :: String -- ^ name of pattern in which it was defined. } -- Fr t d expr pn -- represents an automatic computation, such as * or +. | Fr { fraut :: AutType -- ^ the type of automatic computation , frdec :: Declaration -- ^ where the result is to be stored , frcmp :: Expression -- ^ expression to be computed , frpat :: String -- ^ name of pattern in which it was defined. } -- View this message in context: http://www.nabble.com/Differences-in-pattern-matching-syntax--tp21416338p214... Sent from the Haskell - Glasgow-haskell-users mailing list archive at Nabble.com.