Haskell:
Interpolating a string

How to:

In Haskell, string interpolation isn’t baked in, but with the interpolate package, you can get pretty close. First, ensure you have the package:

cabal update
cabal install interpolate

Now, write some Haskell:

{-# LANGUAGE QuasiQuotes #-}
import Data.String.Interpolate (i)

main :: IO ()
main = do
    let name = "world"
    let greeting = [i|Hello, #{name}!|]
    putStrLn greeting

Run it:

Hello, world!

Deep Dive

Historically, Haskell didn’t come with string interpolation out of the box. It’s a feature more common in scripting languages. Interpolation in Haskell became smoother with the development of quasiquoters, which allow you to define your own custom syntax—like our i for interpolating strings.

Alternatives? Sure, use printf from Text.Printf, or concatenate strings and variables with ++. But these lack the elegance and simplicity of interpolation.

Implementation-wise, interpolate transforms your interpolated strings into regular Haskell strings at compile-time using Template Haskell, so there’s no performance hit when running your code. It’s clever and clean, just like Haskell.

See Also

• Hackage - interpolate package
• Hackage - Text.Printf module
• Haskell Wiki - Quasiquotation
• For rich templating, check out Hackage - Mustache templates