Question

I have found a lot of useful information on using Agda as a proof system. I have found virtually no information on using Agda to write usable programs. I cannot even find a “hello world” example that compiles with the most recent version of Agda.

So,

1. Are there any good tutorials on Agda as a programming language?

2. Are there other languages of a similar nature (lazy functional dependently typed) that have more mature documentation for using them as a programming language? (I found tons of great documentation on Coq, but, again, no “Hello World”).

Answer 1

To print a string in Agda, you need the std lib. You can find a “hello world” example here for Agda 2.2.6 and std lib 0.3. This example doesn’t work for current Agda 2.3.0 and std lib 0.6. I read some sources in std lib 0.6, and find that the following one works:

module hello where

open import IO.Primitive using (IO; putStrLn)
open import Data.String using (toCostring; String)
open import Foreign.Haskell using (Unit)

main : IO Unit
main = putStrLn (toCostring "Hello, Agda!")

To compile it, you need

1. save it to “./hello.agda”
2. download lib-0.6.tar.gz, and unpack it to somewhere, say DIR
3. cd DIR/ffi && cabal install
4. agda -i DIR/src -i . -c hello.agda

On my MacOSX Lion with ghc-7.4.2 and cabal-1.16.0, this example works fine. I get an executable program named “hello” with size 19.1M.