This library is intended to provide a minimal binding to Haskell IO, which respects Agda's semantics while still providing the performance benefits of lazy IO.

The problem with directly binding to Haskell's IO monad is that lazy IO does not respect Agda's semantics. For example, given two programs:

hello1 : List Char -> IO String
hello1 [] = putStr "Hello"
hello1 (x :: xs) = putStr "Hello"

and:

hello2 : List Char -> IO String
hello2 xs = putStr "Hello"

It is routine to prove that hello1 and hello2 are extensionally equal, and so we would expect that:

mainX = getContents >>= helloX

would have the same behaviour, no matter which helloX function is plugged in.

Unfortunately, Haskell program equality does not include eta-equivalence on lists, and so main1 will block waiting for input, where main2 prints "Hello".

This library provides an implementation of a simple transactional model of IO. In this model, all IO has strict semantics, but the programmer has control over the order of evaluation, via use of the commit command. All IO is buffered until a commit operation takes place, so the program:

putStr "Hello, World\n"

prints nothing, whereas the program:

putStr "Hello, World\n" >> commit

prints the expected greeting. If data is read lazily, then it is not made strict until the commit operation. For example, the program System.IO.Examples.DevNull reads and discards its input:

getBytes {lazy} >>= λ bs →
  putStr "Done.\n" >>
  commit

runs in constant space, for example when fed a 500M file:

 559,216,832 bytes allocated in the heap
      91,624 bytes copied during GC
      20,392 bytes maximum residency (1 sample(s))
      33,296 bytes maximum slop
           2 MB total memory in use (0 MB lost due to fragmentation)

Its strict equivalent reads the whole file into memory.

There is also a more experimental library which is intended to provide the benefit of iteratees to Agda IO. It is based on transducers (that is automata that can read input and produce output), and is typed using a variant on session types. Since automata are explicit about when they perform input, they are not subject to eta-equivalence, so can be used without worrying about breaking Agda's semantics.

Currently the transducer library is provided without any bindings to the I/O library -- this will be fixed soon.

Some simple examples are in System.IO.Examples.Transducers.

Agda 2.2.6 or 2.2.8, and the Agda standard library 0.3 or 0.4.

Haskell libraries bytestring and utf8-string.

$ agda -i src -i <agda-stdlib-src> -c src/System/IO/Examples/HelloWorld.agda

$ ./HelloWorld