Hi, I tried Simon's : Gc{} -> case r of { GC{ grspe = x } -> Tm x } This still failed with <<Loop>>, like alternative 1. Then I tried Claus Reinke's suggestion: Gc{} -> Tm $! (grspe r) which had the same result , the <<loop>>. However, I was very fortunate to have Bas Joosten look into this with me. He suggested: {- Alternative 3 : -} antecedent :: Rule -> Expression antecedent r = case r of Ru{rrsrt = AlwaysExpr} -> error ("(Module ADLdef:) illegal call to antecedent of rule "++show r) Ru{} -> rrant r Sg{} -> antecedent (srsig r) Gc{} -> Tm (grspe r) Fr{} -> frcmp r We tried this alternative, and... It doesn't loop, as doesn't alternative 2: {- 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 In my case however, I like the syntax of alternative 3 much more than that of alternative 2. I do not exactly understand why the alternatives 3 and 1 behave differently. It probably has something to do with strictness, but that isn't really my cup of tea. I do wish to express my thanks to Simon, Claus and Daniel Fisher who took the trouble in reacting to my post. Thanks! (And of course Bas for being around at the right time 8-)) {- Alternative 3 : -} antecedent :: Rule -> Expression antecedent r = case r of Ru{rrsrt = AlwaysExpr} -> error ("(Module ADLdef:) illegal call to antecedent of rule "++show r) Ru{} -> rrant r Sg{} -> antecedent (srsig r) Gc{} -> Tm (grspe r) Fr{} -> frcmp r {- 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 This notation is what I was looking for. I still think that -- 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.