string-interpolate
Haskell string/text/bytestring interpolation that just works
https://gitlab.com/williamyaoh/string-interpolate/blob/master/README.md
Version on this page: | 0.3.2.1 |
LTS Haskell 22.33: | 0.3.4.0 |
Stackage Nightly 2024-09-07: | 0.3.4.0@rev:1 |
Latest on Hackage: | 0.3.4.0@rev:1 |
string-interpolate-0.3.2.1@sha256:a2fc9f9fa5b2ba254227164d2218c1665cff5fd3dae750e77163f6a9d7dcbb90,4125
Module documentation for 0.3.2.1
string-interpolate
![license](https://img.shields.io/badge/license-BSD--3-ff69b4.svg)
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?
Behold:
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
.
In addition to the main quasiquoter i
, there are two additional quasiquoters
for handling multiline strings. If you need to remove extra whitespace and
collapse into a single line, use iii
. If you need to remove extra indentation
but keep linebreaks, use __i
.
If you need even more specific functionality in how you handle whitespace,
there are variants of __i
and iii
with different behavior for
surrounding newlines. These are suffixed by either 'E
or 'L
depending
on what behavior you need. For instance, __i'E
will remove extra indentation
from its body, but will leave any surrounding newlines intact. iii'L
will
collapse its body into a single line, and collapse any surrounding newlines
at the beginning/end into a single newline.
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.
Usage
First things first: add string-interpolate to your dependencies:
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!
string-interpolate also needs to know what concrete type it’s interpolating. For instance, the following code won’t work:
showIt :: Show a => a -> String
showIt it = [i|The value: #{it}|]
You would need to convert it
to a String using show
first.
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 interpolate arbitrary expressions:
λ> [i|Tomorrow's date is #{addDays 1 $ utctDay now}.|] :: String
>>> "Tomorrow's date is 2019-03-11."
string-interpolate, by default, handles multiline strings by copying the newline verbatim into the output.
λ> :{
| [i|
| a
| b
| c
| |] :: String
| :}
>>> "\n a\n b\n c\n"
Another quasiquoter, iii
, is provided that handles multiline strings/whitespace
in a different way, by collapsing any whitespace into a single space. The
intention is to use it when you want to split something across multiple
lines in source for readability but want it emitted like a normal sentence.
iii
is otherwise identical to i
, with the ability to interpolate arbitrary values.
λ> :{
| [iii|
| Lorum
| ipsum
| dolor
| sit
| amet.
| |] :: String
| :}
>>> "Lorum ipsum dolor sit amet."
One last quasiquoter, __i
, is provided that handles removing indentation
without removing line breaks, perhaps if you need to output code samples
or error messages. Again, __i
is otherwise identical to i
, with the ability
to interpolate arbitrary values.
λ> :{
| [__i|
| id :: a -> a
| id x = y
| where y = x
| |] :: String
| :}
>>> "id :: a -> a\nid x = y\n where y = x"
The intended mnemonics for remembering what iii
and __i
do:
iii
: Look at the i’s as individual lines which have been collapsed into a single line__i
: Look at the i as being indented
In addition, there are variants of iii
and __i
, desginated by a letter
suffix. For instance, __i'L
will reduce indentation, while collapsing
any surrounding newlines into a single newline.
λ> :{
| [__i'L|
|
| id :: a -> a
| id x = y
| where y = x
|
| |] :: String
| :}
>>> "\nid :: a -> a\nid x = y\n where y = x\n"
Currently there are two variant suffixes, 'E
and 'L
’
'E
: Leave any surrounding newlines intact. To remember what this does, look visually at the capital E; the multiple horizontal lines suggests multiple newlines.'L
: Collapse any surrounding newlines into a single newline. To remember what this does, look visually at the capital L; the single horizontal line suggests a single newline.
Check the Haddock documentation for all the available variants.
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:
- interpolate
- formatting
- Text.Printf, from base
- neat-interpolation
- Interpolation
- interpolatedstring-perl6
Of these, Text.Printf isn’t exception-safe, and neat-interpolation can only produce strict 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.
Features
string-interpolate | interpolate | formatting | Interpolation | interpolatedstring-perl6 | neat-interpolation | |
---|---|---|---|---|---|---|
String/Text support | ✅ | ✅ | ✅ | ⚠️ | ✅ | ⚠️ |
ByteString support | ✅ | ✅ | ❌ | ⚠️ | ✅ | ❌ |
Can interpolate arbitrary Show instances | ✅ | ✅ | ✅ | ✅ | ✅ | ❌ |
Unicode-aware | ✅ | ❌ | ⚠️ | ❌ | ❌ | ⚠️ |
Multiline strings | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
Indentation handling | ✅ | ✅ | ❌ | ✅ | ❌ | ✅ |
Whitespace/newline chomping | ✅ | ❌ | ❌ | ❌ | ❌ | ❌ |
⚠ Since formatting doesn’t support ByteStrings, it technically supports Unicode.
⚠ 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.
⚠ neat-interpolation only supports strict Text. Because of that, it technically supports Unicode.
Performance
Overall: string-interpolate is competitive with the fastest interpolation libraries, only getting outperformed on ByteStrings by Interpolation and interpolatedstring-perl6, and on large, strict Text specifically by formatting.
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
, both Text
types, and both ByteString
types. Numbers are
runtime in relation to string-interpolate; smaller is better.
string-interpolate | formatting | Interpolation | interpolatedstring-perl6 | neat-interpolation | interpolate | |
---|---|---|---|---|---|---|
small String | 1x | 2.8x | 1x | 1x | 1x | |
multi interp, String | 1x | 4.3x | 1x | 1x | 7.9x | |
small Text | 1x | 4.3x | 1.8x | 1.9x | 5.8x | 61x |
multi interp, Text | 1x | 3.5x | 5.3x | 5.3x | 3.3x | 29x |
large Text | 1x | 0.6x | 11x | 11x | 22x | 10,000x |
small lazy Text | 1x | 6.1x | 14.5x | 14.5x | 93x | |
multi interp, lazy Text | 1x | 3.7x | 5.8x | 6x | 34x | |
large lazy Text | 1x | 3.9x | 22,000x | 22,000x | 3,500,000x | |
small ByteString | 1x | 1x | 1x | 47x | ||
multi interp, ByteString | 1x | 0.7x | 0.7x | 17x | ||
large ByteString | 1x | 1x | 1x | 31,000x | ||
small lazy ByteString | 1x | 1x | 1x | 85x | ||
multi interp, lazy ByteString | 1x | 0.4x | 0.4x | 19x | ||
large lazy ByteString | 1x | 0.8x | 0.8x | 1,300,000x |
(We don’t bother running tests on large String
s, because no one is working
with data that large using String
anyways.)
In particular, notice that Interpolation and interpolatedstring-perl6
blow up on both Text types; string-interpolate and formatting have
consistent performance across all benchmarks, with string-interpolation leading
the pack in Text
cases.
All results were tested on an AWS EC2 t2.medium
, with GHC 8.6.5. If you’d
like to replicate the results, the benchmarks are located in bench/
, and can
be run with cabal v2-run string-interpolate-bench -O2 -fextended-benchmarks
.
Larger Text and ByteString
By default, string-interpolate is performance tuned for outputting smaller
strings. If you find yourself regularly needing extremely large outputs, however,
you can change the way output strings are constructed to optimize accordingly.
Enable either the text-builder
or bytestring-builder
Cabal flag, depending
on your need, and you should see speedups constructing large strings, at the
cost of slowing down smaller outputs.
Changes
CHANGELOG
Unreleased
v0.3.2.1 (2023-04-25)
- Added support for GHC 9.6
- Updated dependency versions for HSpec and template-haskell
v0.3.2.0 (2022-12-12)
- Added support for GHC 9.4
v0.3.1.2 (2022-05-15)
- Updated dependency versions for HSpec and template-haskell
v0.3.1.1 (2021-05-21)
- Updated dependency versions for HSpec and template-haskell
v0.3.1.0 (2021-02-12)
- Added variant interpolators,
__i'E
,__i'L
,iii'E
,iii'L
, that handle surrounding newlines in a different way based on suffix.
v0.3.0.2 (2020-10-01)
- Removed upper bounds on
base16-bytestring
. Safe to do so now thattext-converions
has added upper bounds. This change should not affect any users ofstring-interpolate
.
v0.3.0.1 (2020-09-19)
- Downgraded version of
base16-bytestring
to avoid breaking API changes in new versions. (We would just upgrade the version ourselves, but it’s one of our dependenciestext-conversions
that’s not building due to the API changes, not us.)
v0.3.0.0 (2020-06-30)
- Changed the behavior of
iii
to only collapse statically-available whitespace, which also removes the performance penalty of usingiii
. - Removed noise on compile errors if quasiquoters are misused
- Changed behavior of backslash escapes to error on unknown escape characters
v0.2.1.0 (2020-05-04)
- Added
__i
interpolator for stripping indentation in multiline strings - Added benchmarks for lazy Text and lazy ByteString
- Changed default behavior for Text and ByteString to use the actual types
themselves as intermediate objects rather than construct Builders. This
should give significant speedups in the common case of interpolating
smaller outputs. Old behavior can be reenabled using Cabal
flags
-ftext-builder
and-fbytestring-builder
- Gated benchmarks for
Interpolation
andinterpolatedstring-perl6
behind a Cabal flag so that we can still be in Stackage without needing to remove these dependencies
v0.2.0.3 (2020-04-26)
- Commented out
interpolatedstring-perl6
benchmarks, since that library does not build on GHC 8.8.2
v0.2.0.2 (2020-04-25)
- Updated interpolation parser to use enabled language extensions (Thanks Cary Robbins!)
v0.2.0.1 (2020-04-09)
- Fixed bug caused when escaping a backslash right before an interpolation (Thanks Vladimir Stepchenko!)
- Fixed behavior of
iii
(Thanks Vladimir Stepchenko!)
v0.2.0.0 (2019-12-16)
- Added
iii
interpolator for collapsing whitespace/newlines into single spaces - Added feature comparison to/benchmark with
neat-interpolation
- Just generally make the documentation better
- Add homepage info to cabal file/Haddock documentation
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
Builder
s as both sinks and sources - Add support for interpolating Chars without the surrounding quotes
v0.0.1.0 (2019-03-10)
- Initial release