Re: [Haskell-cafe] Best way to build strings?

On Wed, Jul 20, 2005 at 07:00:22PM +0200, Lemmih wrote:

On 7/20/05, Andy Gimblett wrote:

...

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)"

If you only use GHC, you can also implement (or borrow) a type-safe printf using Template Haskell. I think there's some implementation made by Ian Lynagh. Recently I needed to build strings containing shell commands and I also didn't like the ++ approach. I didn't like the printf approach, but rather wanted something more like shell's or Perl's string interpolation, so I could write something like "($p [] $q)" and have $p and $q expanded to values of p and q. I created a small TH library for this (attached). It uses such syntax: $(interp "(%{p} [] %{q})"). I used % because I knew I would often have to use literal $'s. Unfortunately it has some problems. First, TH sometimes doesn't like when I use a global variable in %{ }. I had to work around it by defining additional local helper variables. Second, it would be nice to be able to put arbitrary Haskell expressions inside %{ } - but I couldn't find a Haskell syntax parser producing TH ASTs. There must be some - I guess Template Haskell uses one internally. Best regards Tomasz