[Haskell-cafe] Best way to build strings?
lemmih at gmail.com
Wed Jul 20 13:00:22 EDT 2005
On 7/20/05, Andy Gimblett <A.M.Gimblett at swansea.ac.uk> wrote:
> 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
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 ++ ?
Prelude Text.Printf> printf "(%s  %s)" "hello" "world" :: String
"(hello  world)"
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