On 7/20/05, Andy Gimblett
A small stylistic question: what's the "best" way to build strings containing other values? For example, I have:
data Process = Stop | Prefix String Process | External Process Process
instance Show Process where show Stop = "Stop" show (Prefix l p) = "(" ++ l ++ "->" ++ show p ++ ")" show (External p q) = "(" ++ show p ++ " [] " ++ show q ++ ")"
How about leaving the Show instance automatically derived and defining this instead: showProcess :: Process -> ShowS showProcess Stop = showString "Stop" showProcess (Prefix l p) = showBody (showString l) (showProcess p) showProcess (External p q) = showBody (showProcess p) (showProcess q) showBody :: ShowS -> ShowS -> ShowS showBody a b = showParen True (a . showString " [] " . b)
but to me the extensive use of ++ is not particularly readable.
I'm very fond of Python's interpolation approach, where we'd have something like the following for the External case:
def __str__(self): return "(%s [] %s)" % (self.p, self.q)
which to me seems clearer, or at least easier to work out roughly what the string's going to look like. (The %s does an implicit "convert to string", btw).
Is there a facility like this in Haskell? Or something else I should be using, other than lots of ++ ?
There's Text.Printf: Prelude Text.Printf> printf "(%s [] %s)" "hello" "world" :: String "(hello [] world)" -- Friendly, Lemmih