syntax across languages
Mon, 11 Feb 2002 17:33:35 +0000
> > I'm not sure if "iterate" counts as loop forever?
> I don't think there can be a "loop forever" construct in haskell?
> in OCaml, it is simply:
> let loop(f) = f() ; loop(f)
> and is only useful together with exceptions and side-effects.
and in Haskell, it is simply:
loop f = f >> loop f
or, as there is little difference between control structures and
other data structures, you can convert the latter into the former,
using the list version of loop, called repeat:
loop f = mapM_ id $ repeat f
loop = foldr1 (>>) . repeat
example (get ready to hit that interrupt key;-):
loop (putStrLn "hi")
> > I can't remember any %-based sprintf off the top of my head, but hey,
> > who'd want one anyway?
> me! I've always wondered how you internationalize a program without it?
> "This encryption key is too simple (must be at least %d characters long)"
> "The package %s needs to be installed. Do you want to install it?"
> (I agree you don't need the full power of sprintf for this)
indeed not. you only want to factor out the strings, so that they
can be collected in a single space for the translators/customizers
convenience. functional abstraction allows you to just do that
(then put the functions into a module per language, or collect them
in tuples/records, and link or select as appropriate):
encryptionTooSimple d =
"This encryption key is too simple (must be at least "
++(show (d::Integer))++ characters long)"
packageMissing s =
"The package "++s++" needs to be installed. Do you want to install it?"
but if you absolutely want printf-style formatting, Danvy's solution
is very nice and simple (once someone had the idea;-):
int k r x = k (r++(show (x::Integer))) -- an Integer
str k r x = k (r++x) -- a String
s x k r = k (r++x) -- format String fragment
nl = s "\n" -- newline
format p = p id ""
(similar continuation-based functions for other types)
Now you can try it in Hugs:
Main> :t format $ s "age: " . int . s " name: " . str . nl
format $ s "age: " . int . s " name: " . str . nl :: Integer -> [Char] -> [Char]
Main> format ( s "age: " . int . s " name: " . str . nl ) 10 "IGS"
"age: 10 name: IGS\n"
7 pp. This report supersedes the earlier report BRICS RS-98-5.
Extended version of an article to appear in Journal of Functional
> i don't put any non-simple functions, the main aim is not to show how it can
> be done, but what are the various names used in various languages.
but it is one of the major features of functional languages that
it is so easy to compose/reuse functionality! this is why Haskell
doesn't need a name for everything - composing the functions can
be as simple as (and more flexible than) composing the names.