# string-interpolate [![pipeline status](https://gitlab.com/williamyaoh/string-interpolate/badges/master/pipeline.svg)](https://gitlab.com/williamyaoh/string-interpolate/commits/master) [![hackage version](https://img.shields.io/hackage/v/string-interpolate.svg)](http://hackage.haskell.org/package/string-interpolate) [![license](https://img.shields.io/badge/license-BSD--3-ff69b4.svg)](https://gitlab.com/williamyaoh/string-interpolate/blob/master/LICENSE) 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: ```haskell 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](https://hackage.haskell.org/package/text-conversions). ## Usage First things first: add **string-interpolate** to your dependencies: ```yaml dependencies: - string-interpolate ``` and import the quasiquoter and enable `-XQuasiQuotes`: ```haskell {-# LANGUAGE QuasiQuotes #-} import Data.String.Interpolate ( i ) ``` Wrap anything you want to be interpolated with `#{}`: ```haskell λ> name = "William" λ> [i|Hello, #{name}!|] :: String >>> "Hello, William!" ``` You can interpolate in anything which implements `Show`: ```haskell 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: ```haskell 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. ```haskell 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: ```haskell 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. ```haskell 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. ```haskell 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. ```haskell 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. ```haskell 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: ```haskell λ> [i|\#{ some inner text }#|] :: String >>> "#{ some inner text }#" ``` ## Comparison to other interpolation libraries Some other interpolation libraries available: * [**interpolate**](https://hackage.haskell.org/package/interpolate) * [**formatting**](https://hackage.haskell.org/package/formatting) * **Text.Printf**, from base * [**neat-interpolation**](https://hackage.haskell.org/package/neat-interpolation) * [**Interpolation**](http://hackage.haskell.org/package/Interpolation) * [**interpolatedstring-perl6**](http://hackage.haskell.org/package/interpolatedstring-perl6-1.0.1) 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.