Henning Thielemann
What about dropping Guards? :-) Are they necessary? Do they lead to more readable source code?
Absolutely. In Haskell's syntax, if-then-else-if interacts badly with do notation, and Haskell lacks a direct analogy to Lisp's cond. case () of () | p1 -> e1 | p2 -> e2 ... works beautifully as a replacement. Also, GHC's pattern guards are a nice feature, and frequently seem clearer than case. Compare, e.g., parseCmd ln | Left err <- parse cmd "Commands" ln = BadCmd $ unwords $ lines $ show err | Right x <- parse cmd "Commands" ln = x with the Haskell-98 alternative parseCmd ln = case parse cmd "Commands" ln of Left err -> BadCmd $ unwords $ lines $ show err Right x -> x The trade-off: using pattern guards makes it harder to verify (and ensure) that the exact same expression is being matched against; using case makes it harder to see exactly what is being matched against. Furthermore, guards are an extension of pattern matching, which means you can write code like this: xn !! n | n < 0 = error "Prelude.(!!): Negative index" [] !! n = error "Prelude.(!!): Index overflow" (x:xn) !! n | n == 0 = x (x:xn) !! n = xn !! (n - 1) Exactly one equation for each edge in the control-flow graph, which is nice and not easily done (I'm not sure it's even possible) without guards. Pattern guards are also nice for implementing ‘views’: -- | Convert an 'XMLData' into an equivalent application of -- 'Balanced', if possible. In any case, return an equivalent data -- structure. balance (Balanced es) = Balanced es balance (LeftLeaning (LeftBalanced e:es)) | Balanced es' <- balance (LeftLeaning es) = Balanced (e:es') balance (LeftLeaning []) = Balanced [] balance (RightLeaning [("", "", es)]) = Balanced es balance (RightLeaning []) = Balanced [] balance e = e Where XMLData can store a (nearly) arbitrary fragment of an XML document. The problem being solved by the pattern guard in the second equation is that the data type is ambiguous; there is more than one way to represent a ‘balanced’ XML fragment (that is, the concatenation of a sequence of well-formed XML fragments and CDATA sections). This function attempts to coerce the data structure passed in into a canonical representation; it succeeds if the data is in fact balanced and fails otherwise. The pattern guard illustrates how to use this function as a replacement for pattern matching on Balanced, to catch all cases where the argument is in fact balanced (we can't use it in this case as a replacement for the first equation, since that create an infinite loop, but in other functions we could). I'm sure there are uses I'm forgetting, but I think that's enough.
Do they lead to more efficient code? I could perfectly live without them up to now.
Well, I could never do without them. Jonathan Cast