I would like to use Haskell code in Agda, for example, something like a function that gives back a list of pairs of integers and strings.
I saw this documentation: https://agda.readthedocs.io/en/v2.6.1.1/language/foreign-function-interface.html
But I don't know how to map Haskell tuples to an Agda type, because for example in a mapping like
{-# COMPILE GHC APair = data ?????? #-}
I don't know how to fill the????-s, as I don't have the definition of the tuple data type.
However, pairs are not listed at the builtin pairings either.
How should I proceed?
The standard library does this in Foreign.Haskell.Pair
( https://agda.github.io/agda-stdlib/Foreign.Haskell.Pair.html ). The relevant code is
record Pair (A : Set a) (B : Set b) : Set (a ⊔ b) where
constructor _,_
field fst : A
snd : B
open Pair public
{-# FOREIGN GHC type AgdaPair l1 l2 a b = (a , b) #-}
{-# COMPILE GHC Pair = data MAlonzo.Code.Foreign.Haskell.Pair.AgdaPair ((,)) #-}
There's a little bit of fiddling to account for the universe levels in the Agda type, which don't appear in the Haskell pairs. If you don't need that, this should suffice:
data Pair (A B : Set) : Set where
_,_ : A → B → Pair A B
{-# COMPILE GHC Pair = data (,) ((,)) #-}
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