formatting
Combinator-based type-safe formatting (like printf() or FORMAT)
https://github.com/AJChapman/formatting#readme
Version on this page: | 7.1.3 |
LTS Haskell 22.37: | 7.2.0 |
Stackage Nightly 2024-10-08: | 7.2.0 |
Latest on Hackage: | 7.2.0 |
formatting-7.1.3@sha256:04fcfd730dd11c961ebf921b4271d52913922d9a7e90e0353b718d581d37f38c,3426
formatting
Formatting is a type-safe and flexible library for formatting text from built-in or custom data types.
- Hackage Documentation
- The original blog post introducing the library, but note that some of the types have changed:
Holey
is no longer used, andFormat
’s type has changed tonewtype Format r a = Format {runFormat :: (Builder -> r) -> a}
Usage
You will probably need the OverloadedStrings
language extension, and to import Formatting
:
{-# LANGUAGE OverloadedStrings #-}
import Formatting
You may also need some or all of these:
import qualified Data.Text as T
import qualified Data.Text.Lazy as TL
import qualified Data.Text.Lazy.Builder as TLB
Now a simple example:
> format ("Person's name is " % text % " and age is " % int) "Dave" 54
"Person's name is Dave and age is 54"
In this example, the formatters are two string literals (which take no arguments), and two formatters which take arguments: text
, which takes a lazy Text
, and int
which takes any Integral
, such as Int
.
They are all joined together using the %
operator, producing a formatter which takes two arguments: a lazy Text
and an Integral
.
It produces a lazy Text
, because we used format
.
To produce other string types, or print the result instead, refer to this table:
To produce a | use |
---|---|
TL.Text |
format |
T.Text |
sformat |
Builder |
bformat |
String |
formatToString |
To print the values instead, refer to this table:
To print to | use |
---|---|
stdout |
fprint |
stdout , appending a newline |
fprintLn |
a handle | hprint |
a handle, appending a newline | hprintLn |
Apart from the %
operator, formatters can also be joined using the monoid append operator (<>
) to avoid repeating the same argument, they can be chained using %.
, and there are also formatter combinators for composing more advanced combinators.
More on this below.
Formatter Quick Reference
Built-in formatters:
To format a | e.g. | as | use | short form |
---|---|---|---|---|
lazy Text |
"Hello" |
"Hello" |
text |
t |
strict Text |
"World!" |
"World!" |
stext |
st |
String |
"Goodbye" |
"Goodbye" |
string |
s |
Builder |
"Bathtub" |
"Bathtub" |
builder |
|
Show a => a |
[1, 2, 3] |
"[1, 2, 3]" |
shown |
sh |
Char |
'!' |
"!" |
char |
c |
Integral a => a |
23 |
"23" |
int |
d |
Real a => a |
123.32 |
"123.32" |
float |
sf |
Real a => a |
123.32 |
"123.320" |
fixed 3 |
f |
Scientific |
scientific 60221409 16 |
"6.0221409e23" |
sci |
|
Scientific |
scientific 60221409 16 |
"6.022e23" |
scifmt Exponent (Just 3) |
|
Buildable n, Integral n => n |
123456 |
"12.34.56" |
groupInt 2 '.' |
|
Buildable n, Integral n => n |
12000 |
"12,000" |
commas |
|
Integral n => n |
32 |
"32nd" |
ords |
|
Num a, Eq a => a |
1 |
"1 ant" |
int <> plural "ant" "ants" |
|
Num a, Eq a => a |
2 |
"2 ants" |
int <> plural "ant" "ants" |
|
Enum a => a |
a |
"97" |
asInt |
|
Integral a => a |
23 |
"10111" |
bin |
b |
Integral a => a |
23 |
"0b10111" |
prefixBin |
|
Integral a => a |
23 |
"27" |
oct |
o |
Integral a => a |
23 |
"0o27" |
prefixOct |
|
Integral a => a |
23 |
"17" |
hex |
x |
Integral a => a |
23 |
"0x17" |
prefixHex |
|
Integral a => a |
23 |
"13" |
base 20 |
|
Buildable a => a |
10 |
" 10" |
left 4 ' ' |
l |
Buildable a => a |
10 |
"10 " |
right 4 ' ' |
r |
Buildable a => a |
10 |
" 10 " |
center 4 ' ' |
|
Buildable a => a |
123456 |
"123" |
fitLeft 3 |
|
Buildable a => a |
123456 |
"456" |
fitRight 3 |
|
Buildable a => a |
True |
"True" |
build |
|
a |
undefined |
"gronk!" |
fconst "gronk!" |
Formatter Combinator Quick Reference
Formatter combinators take a formatter and modify it somehow, e.g. by using it to format elements of a list, or changing its output.
Built-in formatter combinators:
To format a | e.g. | as | use |
---|---|---|---|
Maybe a |
Nothing |
"Goodbye" |
maybed "Goodbye" text |
Maybe a |
Just "Hello" |
"Hello" |
maybed "Goodbye" text |
Maybe a |
Nothing |
"" |
optioned text |
Maybe a |
Just "Hello" |
"Hello" |
optioned text |
Either a b |
Left "Error!" |
"Error!" |
eithered text int |
Either a b |
Right 69 |
"69" |
eithered text int |
Either a x |
Left "bingo" |
"bingo" |
lefted text |
Either a x |
Right 16 |
"" |
lefted text |
Either x a |
Right "bingo" |
"bingo" |
righted text |
Either x a |
Left 16 |
"" |
righted text |
Foldable t => t a |
[1, 2, 3] |
"1st2nd3rd" |
concatenated ords |
Foldable t => t a |
[123, 456, 789] |
"789456123" |
joinedWith (mconcat . reverse) int |
Foldable t => t a |
[1, 2, 3] |
"1||2||3" |
intercalated "||" int |
Foldable t => t a |
[1, 2, 3] |
"1 2 3" |
unworded int |
Foldable t => t a |
[1, 2, 3] |
"1\n2\n3" |
unlined d |
Foldable t => t a |
[1, 2, 3] |
"1 2 3" |
spaced int |
Foldable t => t a |
[1, 2, 3] |
"1,2,3" |
commaSep int |
Foldable t => t a |
[1, 2, 3] |
"1st, 2nd, 3rd" |
commaSpaceSep ords |
Foldable t => t a |
["one", "two", "three"] |
"[one, two, three]" |
list t |
Foldable t => t a |
["one", "two", "three"] |
"[\"one\", \"two\", \"three\"]" |
qlist t |
[a] |
[1..] |
"[1, 10, 11, 100]" |
took 4 (list bin) |
[a] |
[1..6] |
"[4, 5, 6]" |
dropped 3 (list int) |
a |
"one two\tthree\nfour |
"one, two, three, four" |
splat isSpace commaSpaceSep stext |
a |
1234567890 |
"[123, 456, 789, 0]" |
splatWith (chunksOf 3) list int |
a |
"one,two,three" |
"one\ntwo\nthree\n" |
splatOn "," unlined t |
a |
"one two three " |
"[one, two, three]" |
worded list text |
a |
"one\n\ntwo\nthree\n\n |
"["one", "", "two", "three", ""]" |
lined qlist text |
a |
123456 |
"654321" |
alteredWith TL.reverse int |
a |
"Data.Char.isUpper |
"DCU" |
charsKeptIf isUpper string |
a |
"Data.Char.isUpper |
"ata.har.ispper" |
charsRemovedIf isUpper string |
a |
"look and boot" |
"leek and beet" |
replaced "oo" "ee" text |
a |
"look and boot" |
"LOOK AND BOOT" |
uppercased |
a |
"Look and Boot" |
"look and boot" |
lowercased |
a |
"look and boot" |
"Look And Boot" |
titlecased |
a |
"hellos" |
"he..." |
ltruncated 5 text |
a |
"hellos" |
"h...s" |
ctruncated |
a |
"hellos" |
"...os" |
rtruncated 5 text |
a |
1 |
" 1" |
lpadded 3 int |
a |
1 |
"1 " |
rpadded 3 int |
a |
1 |
" 1 " |
cpadded 3 int |
a |
123 |
"123 " |
lfixed 4 int |
a |
123456 |
"1..." |
lfixed 4 int |
a |
123 |
" 123" |
rfixed 4 int |
a |
123456 |
"...6" |
rfixed 4 int |
a |
123 |
" 123 " |
cfixed 2 1 ' ' int |
a |
1234567 |
"12...7" |
cfixed 2 1 ' ' int |
a |
"Goo" |
"McGoo" |
prefixed "Mc" t |
a |
"Goo" |
"Goosen" |
suffixed "sen" t |
a |
"Goo" |
"McGooMc" |
surrounded "Mc" t |
a |
"Goo" |
"McGoosen" |
enclosed "Mc" "sen" t |
a |
"Goo" |
"'Goo'" |
squoted t |
a |
"Goo" |
"\"Goo\"" |
dquoted t |
a |
"Goo" |
"(Goo)" |
parenthesised t |
a |
"Goo" |
"[Goo]" |
squared t |
a |
"Goo" |
"{Goo}" |
braced t |
a |
"Goo" |
"<Goo>" |
angled t |
a |
"Goo" |
"`Goo`" |
backticked t |
a |
"Goo" |
" Goo" |
indented 3 t |
Foldable t => t a |
[1, 2, 3] |
" 1\n 2\n 3" |
indentedLines 2 d |
a |
"1\n2\n3" |
" 1\n 2\n 3" |
reindented 2 t |
Integral i, RealFrac d => d |
6.66 |
"7" |
roundedTo int |
Integral i, RealFrac d => d |
6.66 |
"6" |
truncatedTo int |
Integral i, RealFrac d => d |
6.66 |
"7" |
ceilingedTo int |
Integral i, RealFrac d => d |
6.66 |
"6" |
flooredTo int |
field through a Lens' s a |
(1, "goo") |
"goo" |
viewed _2 t |
field through a record accessor s -> a |
(1, "goo") |
"1" |
accessed fst d |
Integral a => a |
4097 |
"0b0001000000000001" |
binPrefix 16 |
Integral a => a |
4097 |
"0o0000000000010001" |
octPrefix 16 |
Integral a => a |
4097 |
"0x0000000000001001" |
hexPrefix 16 |
Ord f, Integral a, Fractional f => a |
1024 |
"1KB" |
bytes shortest |
Ord f, Integral a, Fractional f => a |
1234567890 |
"1.15GB" |
bytes (fixed 2) |
Composing formatters
%.
is like %
but feeds one formatter into another:
λ> format (left 2 '0' %. hex) 10
"0a"
Using more than one formatter on the same argument
λ> now <- getCurrentTime
λ> format (year % "/" <> month <> "/" % dayOfMonth) now
"2015/01/27"
The Buildable Typeclass
One of the great things about formatting
is that it doesn’t rely on typeclasses: you can define one or more formatters for each of your types.
But you also have the option of defining a ‘default’ formatter for a type, by implementing the Buildable
typeclass, which has one method: build :: p -> Builder
.
Once this is defined for a type, you can use the build
formatter (which is distinct from the build
method of Buildable
!):
> format ("Int: " % build % ", Text: " % build) 23 "hello"
"Int: 23, Text: hello"
Note that while this can be convenient, it also sacrifices some type-safety: there’s nothing preventing you from putting the arguments in the wrong order, because both Int
and Text
have a Buildable
instance.
Note also that if a type already has a Show
instance then you can use this instead, by using the shown
formatter.
Understanding the Types
Formatters generally have a type like this:
Format r (a -> r)
This describes a formatter that will eventually produce some string type r
, and takes an a
as an argument.
For example:
int :: Integral a => Format r (a -> r)
This takes an Integral a
argument, and eventually produces an r
.
Let’s work through using this with format
:
-- format has this type:
format :: Format TL.Text a -> a
-- so in 'format int', called with an 'Int', 'int's type specialises to:
int :: Format TL.Text (Int -> TL.Text)
-- and 'format's 'a' parameter specialises to 'Int -> TL.Text':
format :: Format TL.Text (Int -> TL.Text) -> Int -> TL.Text
-- so 'format int' takes an Int and produces text:
format int :: Int -> TL.Text
What can be confusing in the above is that int
’s a
parameter expands to Int
, but format
’s a
parameter expands to Int -> TL.Text
.
Now let’s look at what happens when we use the %
operator to append formatters:
-- Here are the types of the functions we will use:
(%) :: Format r a -> Format r' r -> Format r' a
int :: Format r (Int -> r) -- simplified for this use
stext :: Format r (T.Text -> r)
-- Within the call to '%', in the expression 'int % stext', the type parameters expand like this:
-- r = T.Text -> r'
-- a = Int -> T.Text -> r'
-- and so we have these types:
int :: Format (T.Text -> r') (Int -> T.Text -> r')
stext :: Format r' (T.Text -> r')
int % stext :: Format r' (Int -> T.Text -> r')
-- And so when we use 'format' we get a function that takes two arguments and produces text:
format (int % stext) :: Int -> T.Text -> TL.Text
Comparison with Other Languages
Example:
format ("Person's name is " % text % ", age is " % hex) "Dave" 54
or with short-names:
format ("Person's name is " % t % ", age is " % x) "Dave" 54
Similar to C’s printf
:
printf("Person's name is %s, age is %x","Dave",54);
and Common Lisp’s FORMAT
:
(format nil "Person's name is ~a, age is ~x" "Dave" 54)
Formatter Examples
“Hello, World!”: Texts
> format (text % "!") "Hi!"
"Hi!!"
> format (string % "!") "Hi!"
"Hi!!"
123: Integers
> format int 23
"23"
23.4: Decimals
> format (fixed 0) 23.3
"23"
> format (fixed 2) 23.3333
"23.33"
> format shortest 23.3333
"23.3333"
> format shortest 0.0
"0.0"
> format sci 2.3
"2.3"
> format (scifmt Fixed (Just 0)) 2.3
"2"
1,242: Commas
> format commas 123456778
"123,456,778"
> format commas 1234
"1,234"
1st: Ordinals
> format ords 1
"1st"
> format ords 2
"2nd"
> format ords 3
"3rd"
> format ords 4
"4th"
3F: Hex
> format hex 15
"f"
> format hex 25
"19"
Monday 1st June: Dates & times
> now <- getCurrentTime
> later <- getCurrentTime
> format (dayOfMonth % "/" % month % "/" % year) now now now
"16/06/2014"
> format day now
"167"
> format hms now
"08:24:41"
> format tz now
"+0000"
> format datetime now
"Mon Jun 16 08:24:41 UTC 2014"
> format century now
"20"
> format (dayOfMonthOrd % " of " % monthName) now now
"16th of June"
3 years ago: Time spans
> format (diff False) (diffUTCTime later now)
"2 seconds"
> format (diff True) (diffUTCTime later now)
"in 2 seconds"
> format (diff True) (diffUTCTime now later)
"2 seconds ago"
> format (seconds 0 % " secs") (diffUTCTime now later)
"2 secs"
> let Just old = parseTime defaultTimeLocale "%Y" "1980" :: Maybe UTCTime
> format (years 0) (diffUTCTime now old)
"34"
> format (diff True) (diffUTCTime now old)
"in 35 years"
> format (diff True) (diffUTCTime old now)
"35 years ago"
> format (days 0) (diffUTCTime old now)
"12585"
> format (days 0 % " days") (diffUTCTime old now)
"12585 days"
File sizes
> format (bytes shortest) 1024
"1KB"
> format (bytes (fixed 2 % " ")) (1024*1024*5)
"5.00 MB"
Scientific
If you’re using a type which provides its own builder, like the
Scientific
type:
import Data.Text.Lazy.Builder.Scientific
scientificBuilder :: Scientific -> Builder
formatScientificBuilder :: FPFormat -> Maybe Int -> Scientific -> Builder
Then you can use later
easily:
> format (later scientificBuilder) 23.4
"23.4"
Actually, there are now already two handy combinators (sci
and
scifmt
) for the Scientific
type as shown above in the Decimals
section.
Writing your own Formatters
You can include things verbatim in the formatter:
> format (now "This is printed now.")
"This is printed now."
Although with OverloadedStrings
you can just use string literals:
> format "This is printed now."
"This is printed now."
You can handle things later which makes the formatter accept arguments:
> format (later (const "This is printed later.")) ()
"This is printed later."
The type of the function passed to later
should return an instance
of Monoid
.
later :: (a -> Builder) -> Format r (a -> r)
The function you format with (format
, bprint
, etc.)
will determine the monoid of choice. In the case of this library, the
top-level formating functions expect you to build a text Builder
:
format :: Format Text a -> a
Because builders are efficient generators.
So in this case we will be expected to produce Builders from arguments:
format . later :: (a -> Builder) -> a -> Text
To do that for common types you can just re-use the formatting library and use bprint:
λ> :t bprint
bprint :: Format Builder a -> a
> :t bprint int 23
bprint int 23 :: Builder
Coming back to later
, we can now use it to build our own printer
combinators:
> let mint = later (maybe "" (bprint int))
> :t mint
mint :: Integral a => Format r (Maybe a -> r)
Now mint
is a formatter to show Maybe Integer
:
> format mint (readMaybe "23")
"23"
> format mint (readMaybe "foo")
""
Although a better, more general combinator might be:
> let mfmt x f = later (maybe x (bprint f))
Now you can use it to maybe format things:
> format (mfmt "Nope!" int) (readMaybe "foo")
"Nope!"
Hacking
Building with Nix
See README-nix.md.
Running the Tests
From within your nix-shell
, run cabal test
.
The tests are in test/Spec.hs
.
Running the Benchmarks
Start nix-shell
like this: nix-shell --arg doBenchmark true
.
From within your nix-shell
, run cabal bench
.
To build the html benchmarking reports, run cabal bench --benchmark-option=-obench/reports/7.2.0.html > bench/reports/7.2.0.txt
, replacing ‘7.2.0’ with the current version.
This will output the file bench/reports/7.2.0.html
which you can open in a browser, and bench/reports/7.2.0.txt which you can view in a terminal or text editor.
The benchmarks are in bench/bench.hs
.
Changes
7.1.3
- Fix the GHCJS build by not using
double-conversion
, as it relies on a native C library which obviously isn’t available in GHCJS (it is still used in native builds).
7.1.2
- Removed direct dependency on integer-gmp, instead using very similar code from the
text
package. This changed the implementation ofbuild
forInteger
, which results in better performance in some cases, and no performance degradation. See the benchmarking reports in bench/reports for more details. - formatting now compiles on GHCJS (due to the change above).
- Added some benchmarking, starting based on code from the
string-interpolate
package. - Added INLINE pragmas to many very short functions. Results in better performance in the benchmarks.
7.1.1
- Added
charsKeptIf
andcharsRemovedIf
.
7.1.0
- Added common container formatter combinators:
maybed
,optioned
,eithered
,lefted
, andrighted
.
7.0.0.2
- Removed unnecessary dependencies on array and bytestring
- Actually removed code to support GHC < 8.4
7.0.0.1
- Added README.md to extra-source-files so it shows up on Hackage
7.0.0
- Introduced
Formatting.Combinators
. - Fixed: #62 and #60: incorrect formatting of Integral types that do not have negative values (e.g. Word)
- Fixed: #59 rendering of floats e.g. 0.01 as “0.01” rather than “1.0e-2”
- Added dependency of double-conversion to provide fast and correct rendering of floating-point numbers (including the fix for #59).
- Make compatible with bytestring-0.11.0.0
- Removed -O2 ghc flag
- Updated .cabal file version from 1.8 to 2.4
- Drop support for GHC < 8.4
6.3.7
- Introduced instance
Buildable a => Buildable [a]
.
6.3.6
- Bring back
int :: Integral a => Format r (a -> r)
6.3.5
- Avoid pointless conversions on Float/Double.
6.3.3
- The
Data.Text.Format
hierarchy was reexported asFormatting.Internal.Raw
.
6.3.1
- Proper GHC 7.10 -> GHC 8.4 support
6.3.0
- Folded the
text-format
package into this package, removed thedouble-conversion
dependency. Lost the following functions in this:prec
expt
- Added a test suite with regression tests:
- Fixed: #31
- Fixed: #28
- Fixed: https://github.com/bos/text-format/issues/18
6.2.5
- Changed microseconds to display as “us” to avoid unicode issues.
6.2.1
- Added bytesDecimal
6.2.0
- Dropped Holey/HoleyT in favour of simpler Format type.
- Added Monoid instance.
- Added back Category instance.
- Dropped Functor instance.
6.1.1
- Add support for GHC 7.10 (time update).
6.1.0
- Add formatter for TimeSpec.
6.0.0
- Changed the type of
Format
. Now you writeFormat r (a -> r)
instead ofFormat a
. - Add
formatToString
function.