Haskell string/text/bytestring interpolation that just works

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BSD-3-Clause licensed by William Yao
Maintained by [email protected]
This version can be pinned in stack with:string-interpolate-,1999

Module documentation for

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Haskell having 5 different textual types in common use (String, strict and lazy Text, strict and lazy ByteString) means that doing any kind of string manipulation becomes a complicated game of type tetris with constant conversion back and forth. What if string handling was as simple and easy as it is in literally any other language?


showWelcomeMessage :: Text -> Integer -> Text
showWelcomeMessage username visits =
  [i|Welcome to my website, #{username}! You are visitor #{visits}!|]

No more needing to mconcat, mappend, and (<>) to glue strings together. No more having to remember a gajillion different functions for converting between strict and lazy versions of Text, or having to worry about encoding between Text <=> ByteString. No more getting bitten by trying to work with Unicode ByteStrings. It just works!

string-interpolate provides a quasiquoter, i, that allows you to interpolate expressions directly into your string. It can produce anything that is an instance of IsString, and can interpolate anything which is an instance of Show.

Unicode handling

string-interpolate handles converting to/from Unicode when converting String/Text to ByteString and vice versa. Lots of libraries use ByteString to represent human-readable text, even though this is not safe. There are lots of useful libraries in the ecosystem that are unfortunately annoying to work with because of the need to generate ByteStrings containing application-specific info. Insisting on explicitly converting to/from UTF-8 in these cases and handling decoding failures adds lots of syntactic noise, when often you can reasonably assume that a given ByteString will, 95% of the time, contain Unicode text. So string-interpolate aims to provide reasonable defaults around conversion between ByteString and real textual types so that developers don’t need to constantly be aware of text encodings.

When converting a String/Text to a ByteString, string-interpolate will automatically encode it as a sequence of UTF-8 bytes. When converting a ByteString to String/Text, string-interpolate will assume that the ByteString contains a UTF-8 string, and convert the characters accordingly. Any invalid characters in the ByteString will be converted to the Unicode replacement character � (U+FFFD).

Remember: string-interpolate is not designed for 100% correctness around text encodings, just for convenience in the most common case. If you absolutely need to be aware of text encodings and to handle decode failures, take a look at text-conversions.


First things first: add string-interpolate to your dependencies:

  - string-interpolate

and import the quasiquoter and enable -XQuasiQuotes:

{-# LANGUAGE QuasiQuotes #-}

import Data.String.Interpolate ( i )

Wrap anything you want to be interpolated with #{}:

λ> name = "William"
λ> [i|Hello, #{name}!|] :: String
>>> "Hello, William!"

You can interpolate in anything which implements Show:

λ> import Data.Time
λ> now <- getCurrentTime
λ> [i|The current time is #{now}.|] :: String
>>> "The current time is 2019-03-10 18:58:40.573892546 UTC."

…and interpolate into anything which implements IsString.

string-interpolate must know what concrete type it’s producing; it cannot be used to generate a IsString a => a. If you’re using string-interpolate from GHCi, make sure to add type signatures to toplevel usages!

Strings and characters are always interpolated without surrounding quotes.

λ> verb = 'c'
λ> noun = "sea"
λ> [i|We went to go #{verb} the #{noun}.|] :: String
>>> "We went to go c the sea."

You can also interpolate arbitrary expressions:

λ> [i|Tomorrow's date is #{addDays 1 $ utctDay now}.|] :: String
>>> "Tomorrow's date is 2019-03-11."

Backslashes are handled exactly the same way they are in normal Haskell strings. If you need to put a literal #{ into your string, prefix the pound symbol with a backslash:

λ> [i|\#{ some inner text }#|] :: String
>>> "#{ some inner text }#"

Comparison to other interpolation libraries

Some other interpolation libraries available:

Of these, Text.Printf isn’t exception-safe, and neat-interpolation can only produce Text values. interpolate, formatting, Interpolation, and interpolatedstring-perl6 provide different solutions to the problem of providing a general way of interpolating any value, into any kind of text.


string-interpolate interpolate formatting Interpolation interpolatedstring-perl6
String/Text support ⚠️
ByteString support ⚠️
Can interpolate arbitrary Show instances
Multiline strings

Interpolation supports all five textual formats, but doesn’t allow you to mix and match; that is, you can’t interpolate a String into an output string of type Text, and vice versa.


Overall: string-interpolate is competitive with the fastest interpolation libraries, only getting outperformed by Interpolation and interpolatedstring-perl6, and even then mostly on ByteStrings. Since these two libraries don’t handle Unicode and string-interpolate converts things to UTF-8, some slowdown is to be expected here.

We run three benchmarks: small string interpolation (<100 chars) with a single interpolation parameter; small strings with multiple interpolation parameters, and large string (~100KB) interpolation. Each of these benchmarks is then run against String, strict Text, and strict ByteString.

string-interpolate interpolate formatting Interpolation interpolatedstring-perl6
small String 1x 1x 2x 1x 1x
multi interp, String 1x 7x 2.3x 0.63x 0.63x
small Text 1x 28x 1.5x 2.2x 2.2x
multi interp, Text 1x 22x 1.6x 2.9x 2.9x
large Text 1x 30,000x 1x 80x 80x
small ByteString 1x 15x N/A 0.35x 0.35x
multi interp, ByteString 1x 10x N/A 0.5x 0.5x
large ByteString 1x 100,000x N/A 1.6x 1.6x

(We don’t bother running tests on large Strings, because no one is working with data that large using String anyways.)

In particular, notice that Interpolation and interpolatedstring-perl6 blow up on large Text; string-interpolation and formatting have consistent performance across all benchmarks, with string-interpolation leading the pack in Text cases.

All results were tested on my local machine. If you’d like to attempt to replicate the results, the benchmarks are in bench/ and can be run with a simple stack bench.

(NB: If you’re attempting to reproduce these benchmarks, note that the benchmarks for Interpolation are commented out by default, due to that package not being in Stackage.)




v0.1.0.1 (2019-05-06)

  • Remove Interpolation from the default benchmarks because it’s not on Stackage

v0.1.0.0 (2019-03-17)

  • Add support for using Text and ByteString Builders as both sinks and sources
  • Add support for interpolating Chars without the surrounding quotes

v0.0.1.0 (2019-03-10)

  • Initial release