GHC - Infixl declaration in Haskell

Question

Hi I'm building my own version of a GPU programming Haskell DSL which is called Accelerate. The question is about the the infixl declaration:

Here is the code snippet:

infixl 3 :.
data tail :. head = tail :. head
    deriving (Eq, Show)

I think this snippet is quite simple and clear, but when I was trying to load this into ghci, it failed:

It reported:

Illegal declaration of a type or class operator ‘:.’
      Use TypeOperators to declare operators in type and declarations

Do you have any idea about this problem? The ghc version I'm using is:

The Glorious Glasgow Haskell Compilation System, version 7.8.3

Thank you!

It looks like you're trying to add a heterogeneous list type to haskell; That already exists in the HList package, and is also provided by GHC if you enable `DataKinds`. For example you could have a type `[Int,String,Int]` with kind `[*]`. — Cubic, Nov 12 '14 at 16:45
@Cubic Actually I dont really understand the meaning of that sentence ... However, I agree with your idea that this one is a heterogeneous list. — VELVETDETH, Nov 13 '14 at 05:14

score 9 · Accepted Answer · edited May 23 '17 at 12:22

9

You need

{-# LANGUAGE TypeOperators #-}

in your source file. That is what the error message says. To use them in ghci, you have to enable there as well. See XTypeOperators extension doesn't work as pragma

edited May 23 '17 at 12:22

Community

answered Nov 12 '14 at 11:53

Franky

Oh it fixed! Thanks! But do you have any idea why this pragma should be there? I mean why ghc doesnt support it directly? – VELVETDETH Nov 12 '14 at 11:56
I did not read the [haskell 2010 report](https://www.haskell.org/onlinereport/haskell2010/), but I bet type operators are not included. Without pragmas, ghc complies to the standard. – Franky Nov 12 '14 at 12:06
Oh I see, so it's all about language standard... Thx – VELVETDETH Nov 12 '14 at 12:16
Where should infixr/infixl/infix declaration be put by convention? What if the operator is defined as part of a GADT? I see it could be put anywhere in the page? – CMCDragonkai Dec 28 '14 at 03:23

1 Answers1