See also Foreign Function Interface.


GHC Backend

The GHC backend translates Agda programs into GHC Haskell programs.


The GHC backend can be invoked from the command line using the flag --compile or --ghc:

agda --compile [--compile-dir=<DIR>] [--ghc-flag=<FLAG>]
  [--ghc-strict-data] [--ghc-strict] <FILE>.agda

When the flag --ghc-strict-data is used, inductive data and record constructors are compiled to constructors with strict arguments. (This does not apply to certain builtin types—lists, the maybe type, and some types related to reflection—and might not apply to types with COMPILE GHC = data pragmas.)

When the flag --ghc-strict is used, the GHC backend generates mostly strict code. Note that functions might not be strict in unused arguments, and that function definitions coming from COMPILE GHC pragmas are not affected. This flag implies --ghc-strict-data, and the exceptions of that flag applies to this flag as well. (Note that this option requires the use of GHC 9 or later.)


--compile, --ghc

Compile to GHC Haskell placing the files in subdirectory MAlonzo or the directory given by --compile-dir. Then invoke ghc (or the compiler given by --with-compiler) on the main file, unless option --ghc-dont-call-ghc is given.


Only produce Haskell files, skip the compilation to binary.


Pass flag GHC-FLAG to the Haskell compiler. This option can be given several times.


Compile Agda constructor to strict Haskell constructors.


Generate strict Haskell code.



The following “Hello, World!” example requires some Built-ins and uses the Foreign Function Interface:

module HelloWorld where

open import Agda.Builtin.IO
open import Agda.Builtin.Unit
open import Agda.Builtin.String

  putStrLn : String  IO {-# FOREIGN GHC import qualified Data.Text.IO as Text #-}
{-# COMPILE GHC putStrLn = Text.putStrLn #-}

main : IO ⊤
main = putStrLn "Hello, World!"

After compiling the example

agda --compile HelloWorld.agda

you can run the HelloWorld program which prints Hello, World!.


Frequent error when compiling: Float requires the ieee754 haskell library. Usually cabal v1-install ieee754 or cabal v2-install --lib ieee754 in the command line does the trick.

JavaScript Backend

The JavaScript backend translates Agda code to JavaScript code.


The JavaScript backend can be invoked from the command line using the flag --js:

agda --js [--js-optimize] [--js-minify] [--compile-dir=<DIR>] <FILE>.agda

The --js-optimize flag makes the generated JavaScript code typically faster and less readable.

The --js-minify flag makes the generated JavaScript code smaller and less readable.

Agda can currently generate either CommonJS (used by NodeJS) flavour modules or AMD (for in-browser usage) flavour modules which can be toggled by --js-cjs (default) and --js-amd flags.



Compile to JavaScript, placing translation of module M into file jAgda.M.js. The files will be placed into the root directory of the compiled Agda project, or into the directory given by --compile-dir.


Produce AMD style modules.


Produce CommonJS style modules. This is the default.


Produce minified JavaScript (e.g. omitting whitespace where possible).


Produce optimized JavaScript.


Except for the main module, run the generated modules through node, to verify absence of syntax errors.


Builtin natural numbers

Builtin natural numbers are represented as arbitrary-precision integers. The builtin functions on natural numbers are compiled to the corresponding arbitrary-precision integer functions.

Note that pattern matching on an Integer is slower than on an unary natural number. Code that does a lot of unary manipulations and doesn’t use builtin arithmetic likely becomes slower due to this optimization. If you find that this is the case, it is recommended to use a different, but isomorphic type to the builtin natural numbers.

Irrelevant fields and constructor arguments

Record fields and constructor arguments marked irrelevant or runtime irrelevant are completely erased from the compiled record or data type. For instance,

postulate Parity : Nat  Set

record PNat : Set where
    n    : Nat
    .p   : Parity n
    @0 q : Parity (suc n)

gets compiled by the GHC backend to (up to naming)

newtype PNat = PNat'constructor Integer

Erasable types

A data type is considered erasable if it has a single constructor whose arguments are all erasable types, or functions into erasable types. The compilers will erase

  • calls to functions into erasable types

  • pattern matches on values of erasable type

At the moment the compilers only have enough type information to erase calls of top-level functions that can be seen to return a value of erasable type without looking at the arguments of the call. In other words, a function call will not be erased if it calls a lambda bound variable, or the result is erasable for the given arguments, but not for others.

Typical examples of erasable types are the equality type and the accessibility predicate used for well-founded recursion:

data _≡_ {a} {A : Set a} (x : A) : A  Set a where
  refl : x  x

data Acc {a} {A : Set a} (_<_ : A  A  Set a) (x : A) : Set a where
  acc : ( y  y < x  Acc _<_ y)  Acc _<_ x

The erasure means that equality proofs will (mostly) be erased, and never looked at, and functions defined by well-founded recursion will ignore the accessibility proof.