Frege (programming language)

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Frege logo.svg
Paradigm functional, lazy/non-strict
Designed by Ingo Wechsung
First appeared 2011; 4 years ago (2011)
Stable release 3.22 / January 2015; 9 months ago (2015-01)
Typing discipline static, strong, inferred
OS Cross-platform
License BSD
Filename extensions .fr
Influenced by

Frege is a non-strict, pure functional programming language for the Java virtual machine in the spirit of Haskell.

It is considered a Haskell dialect or simply "a" Haskell for the Java virtual machine.[1]

Frege has a strong static type system with type inference. Higher rank types are supported, though type annotations are required for that.[1] Frege programs are compiled to Java bytecode and run in a Java virtual machine. Existing Java classes and methods can be used seamlessly from Frege after their types have been properly declared.

The language was designed by Ingo Wechsung,[2] who named it after the German mathematician, logician and philosopher Gottlob Frege.

(Unrelated to the Frege Program Prover).

Comparison with Haskell[edit]

A summary of differences between Frege and Haskell is listed at the Differences between Frege and Haskell.

The type String is custom defined as an interface with Java strings. String (++) is bound to Java's String (+).[3] Conversion functions to Haskell correspondent:

packed :: [Char] -> String
unpacked :: String -> [Char]


-- boolean literals true false are not capitalized

Frege's Monad class does not include the method fail, included in a separate class MonadFail.[4]

Numeric classes for floating point types are also different. Haskell's classes Fractional, RealFrac, RealFloat and Floating are not defined. Haskell's class Real defines toRational while Frege's defines (/):[3]

class Real (Num r) => r where     -- classname precedes context
    --- the division operator
    (/) :: r -> r -> r

Hello World program[edit]

-- file
module Hello where  -- moduleName maybe hierarchical as pkgdir.JavaClassname

main args = println $ "Hello world! your arguments are: " ++ show args

Compiling Frege programs[edit]

Frege requires Java-7 JDK or higher to compile and run.

At the console[edit]

As the "Getting started" page states,[5] to compile it:

$ mkdir classes
$ java -Xss1m -jar ${install_dir}/fregec.jar -d classes src/

This assumes the downloaded frege3.xx.vvv.jar has been renamed to fregec.jar for ease of use.

To run the compiled program specify the package name as start class. On GNU/Linux and other Unix systems:

$ java -cp classes:${install_dir}/fregec.jar Hello arg1 arg2
Hello world! your arguments are: ["arg1", "arg2"]

On Microsoft Windows the classpath separator has to be changed to ';'

At the Eclipse devel. environment[edit]

There is a plug-in for Eclipse with instructions given at How-to EclipseFregIDE.

More involved examples[edit]


data Person = P { name :: String, birthyear :: Int }

frege = P "Gottlob Frege" 1848 
smith = P { birthyear = 1990, name = "Joe Smith" }

-- tell if first person is older than second
older :: Person -> Person -> Bool
older P{birthyear} p2 = birthyear < p2.birthyear

main _ = println (frege `older` smith)  -- prints true

Unlike in Haskell, the record fields do not appear in the global namespace. Thus it is possible to reuse the same field name in different types. The accessor functions in the example are known as and Person.birthyear

The record syntax is really syntactic sugar, and the associated data constructor can be used with traditional or record syntax. The same holds for patterns.

Record pattern syntax allows to check for a given constructor and check specific fields or bind them to local variables. This makes patterns independent from the number and order of fields in a constructor.

Using Java classes and methods[edit]

    This program displays the
    current time on standard output
    every other second.
module examples.CommandLineClock where

data Date = native java.util.Date where
    native new :: () -> IO (MutableIO Date)             -- new Date()
    native toString :: Mutable s Date -> ST s String    -- d.toString()

--- 'IO' action to give us the current time as 'String'
current :: IO String
current = do
    d <- ()  -- reads system timer, hence IO

main args =  
    forever do
        current >>= print   -- print formatted date
        print "\r"          -- followed by carriage return
        stdout.flush        -- make sure it's shown
        Thread.sleep 999L   -- wait 0.999 seconds


External links[edit]

In depth[edit]