custom-interpolation

This library provides tools for easily generating string interpolation quasiquoters. The interpolation behavior is customizable and multiple different interpolation methods may be used in a single quasiquoter.

Usage Examples

Example 1: Multiple interpolators

Let's define a basic string interpolation quasiquoter that

• interpolates any Haskell expressions using {} and
• shows the first 10 elements of a list using @{}:

i = interpolateQQ simpleConfig
  { handlers = [ simpleInterpolator {prefix = ""}
               , (applyInterpolator [|show . take 10|]) {prefix = "@"} ] }

>>> [i|2^10 = {show (2 ^ 10)}. Some Fibonacci numbers: @{let x = (\fibs -> 1 : 1 : zipWith (+) fibs (tail fibs)) x in x}.|]
"2^10 = 1024. Some Fibonacci numbers: [1,1,2,3,5,8,13,21,34,55]."

Example 2: SQL substitution

Now for a more complicated example; defining an SQL query quasiquoter that prevents SQL injection.

We can achieve this by replacing expressions between {} with ? and accumulating the actual expression in the first output of the Interpolator handler. This allows us to then apply some SQL library function to the string and the accumulated expressions which takes care of the actual substitution.

import Language.Haskell.TH (appE, listE, Exp, Q)

-- Need an existential type to wrap the differently typed interpolated expressions
data SQLData = forall a. Show a => SQLData a
instance Show SQLData where show (SQLData x) = show x

-- Dummy function that would normally run the query
runSQL sql ds = (sql, ds)

-- The quasiquoter itself
consumeInterpolated :: ([Q Exp], Q Exp) -> Q Exp
consumeInterpolated (exprs, strExpr) = appE (appE [|runSQL|] strExpr) (listE (map (appE [|SQLData|]) exprs))

sql = interpolateQQ defaultConfig
    { finalize = consumeInterpolated,
      handlers = [simpleInterpolator { handler = (\q -> (q, [|"?"|])) }]
    }

>>> [sql|SELECT * FROM user WHERE id = {(11 ^ 5)} AND lastName = {"Smith"}|]
("SELECT * FROM user WHERE id = ? AND lastName = ?",[161051,"Smith"])

Acknowledgements

The CustomInterpolation.Parser module was derived from the here package.