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Unit conversion and manipulation library for Haskell.
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Unit conversion and manipulation library for Haskell.


Check out the docs on Hackage.


We provide a string-parsing constructor to create quantities. This constructor is built using an expression parser, so arithmetic expressions can be used.

>>> fromString "25 m/s"
Right 25.0 meter / second
>>> fromString "fakeunit"
Left (UndefinedUnitError "fakeunit")
>>> fromString "ft + 12in"
Right 2.0 foot

fromString also supports unit conversions, by placing "=>" in between two valid expressions.

>>> fromString "min => s"
Right 60.0 second
>>> fromString "2 ft + 6 in => ft"
Right 2.5 foot

The convert function is used for unit conversions.

computation :: Either QuantityError Quantity
computation = do
  m  <- fromString "30 m"
  ft <- units <$> fromString "ft"
  convert m ft
>>> computation
Right 98.42519685039369 foot


An executable is included called quantities, which is an interface to the fromString function.

$ quantities -h
Usage: quantities [-vh] expression
$ quantities "10 bbl/sec => m^3/min"
95.39237695680004 meter ** 3 / minute
$ quantities "fakeunit"
UndefinedUnitError "fakeunit"

What units are defined?

We have already defined an extensive list of units and SI prefixes. To view them, check out this source file.

There is also functionality to modify this file, or create a totally new one.

How to Contribute

Head over to the Github repo to report an issue or create a patch. Also, non-code contributions are always welcome, especially in this early stage of development. That includes, but is not limited to:

  • API changes/suggestions
  • Code style suggestions
  • Unclear documentation

Don't feel shy to raise an issue! Any constructive criticism helps.


  • Handle temperature units. Simple temperature conversions are easy (celsius to farenheit), but compound units with temperatures are tougher.
  • Add ability to define units out of order; base quantity does not already have to be defined, as long as it is defined in the file.
  • Allow plural form of units. Try to find plural only if no other units can be parsed, or else "ms" will be a plural for meters.
  • Allow use of " to " instead of "=>". Because of parser architecture, need to detect either of these tokens before actually parsing units. Otherwise, it will think "to" is a bad unit.
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