syntax...(strings/interpolation/here docs)

Malcolm Wallace
Wed, 13 Feb 2002 13:42:04 +0000

> Does anybody with their elbows in the
> code think variable interpolation and/or
> multi-line strings are good/doable ideas? 
> Would this be the sort of change one could make
> without a lot of previous familiarity with
> the implementation of Hugs/Ghc?

I don't think it is really necessary to add the feature to the language,
since you can program something very similar for yourself in user-code.

Here's a sample of a single-character macro expansion within strings,
that took a couple of minutes to write.

    module Printf where
    import Maybe(fromMaybe)

    hereDocument :: (Show a, Show b) => a -> b -> String
    hereDocument v w =
        "Multi-line string starts here\
    \    and uses string gap delimiters.\n\
    \    Variable names like $$a are filled in from the\n\
    \    bindings specified in the `with` clause,\n\
    \    e.g. $$a = $a, $$b = $b\n\
    \    and unused bindings pass through e.g. $$c = $c."
               `with` [('a',show v), ('b',show w)]

    with :: String -> [(Char,String)] -> String
    [] `with` env           = []
    ('$':'$':cs) `with` env = '$': cs `with` env
    ('$':c:cs)   `with` env = s ++ cs `with` env
                              where s = fromMaybe ('$':c:[]) (lookup c env)
    (c:cs)       `with` env =  c : cs `with` env