How do I represent a tuple in dhall?

Question

I would like to represent IPv4 addresses in dhall, so I can manage my host configurations.

By default, this is held as Text; but that's clearly unsatisfactory as it allows any old text to slip through. I would like to keep these values as a 4-tuple of 8-bit values.

I don't think that Dhall can allow this natively - the nearest I can see is a record of { a : Natural, b : Natural }, etc., but that's syntactically clunky and still allows for octet values outside of 0-255.

Assuming that I can't achieve this directly in Dhall, perhaps I can define a type in Haskell that can automatically read values that are 4-length lists of Naturals from Dhall,

My questions are:

1. Am I right to think that doing this directly in Dhall is impossible or disproportionately hard?
2. To define this type in Haskell, do I define an instance of Interpret; and if so, how do I define an instance that will read in a 4-part list of Integers, while giving useful error messages for mis-constructed (lists of the wrong length, lists of non-integers or non-lists) or out-of-bounds values (integers that aren't between 0 & 255 inclusive).

This is what I've tried:

{-# LANGUAGE DeriveGeneric   #-}
{-# LANGUAGE RecordWildCards #-}

import Control.Applicative  ( empty, pure )
import Dhall  ( Generic, Interpret( autoWith ), Type( Type, extract, expected ) )
import Dhall.Core  ( Expr( Natural, NaturalLit ) )
import Data.Word  ( Word8 )

newtype IP = IP (Word8, Word8, Word8, Word8)
  deriving Generic

word8 :: Type Word8
word8 = Type {..}
  where
    extract (NaturalLit n) | n >= 0 && n <= 255 = pure (fromIntegral n)
    extract  _             = empty

    expected = Natural

instance Interpret Word8 where
  autoWith _ = word8

instance (Interpret a,Interpret b,Interpret c,Interpret d) => Interpret (a,b,c,d)

instance Interpret IP where

But I'm struggling to find a way to express a value in dhall that can be read in:

λ> input auto "{ _1 = 1, _2 = 2, _3 = 3, _4 = 5 } : { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural}" :: IO IP
*** Exception: 
Error: Expression doesn't match annotation

{ _1 = 1, _2 = 2, _3 = 3, _4 = 5 } : { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural}

(input):1:1

(I'd much rather express an IP as, say, [1,2,3,4]; but following the error messages and the doc for pair seemed to suggest that the numbered record is the way to go).

Is there a way to achieve what I'm after?

Answer 1

For IP addresses, I'd recommend representing them as Dhall strings in the absence of language support for the type. There are two main reasons I suggest this:

• If the language ever does natively support IP addresses then that will provide the smoothest migration path for your users (just drop the quotes)
• In general, there will always exist data types that the language can't perfectly model to make invalid states unrepresentable. If the data type fits well into Dhall's type system then take advantage of that but if it doesn't then don't force it otherwise you'll frustrate yourself and your users. Dhall doesn't have to be perfect; just better than YAML.

For example, if this were a question about native support for dates/times, I'd give the same answer (for the same reasons).

That said, I'll still help debug the issue you ran into. The first thing I did was to attempt to reproduce the issue using a newer version of the dhall package since that has improved error messages:

*Main Dhall> input auto "{ _1 = 1, _2 = 2, _3 = 3, _4 = 5 } : { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural}" :: IO IP
*** Exception: 
Error: Expression doesn't match annotation

{ + _2 : …
, + _3 : …
, + _4 : …
,   _1 : - { … : … }
         + Natural
}

{ _1 = 1, _2 = 2, _3 = 3, _4 = 5 } : { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural} : { _1 : { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural } }
(input):1:1

The error message now shows a "type diff" which explains how the two types differ. In this case the diff already hints at the problem, which is that there is one extra record wrapping the type. It thinks there should only be a single _1 field at the outermost level and the four _1/_2/_3/_4 fields we expected are probably nested inside that field (which is why it thinks that the _1 field stores a record instead of a Natural).

However, we can ask for more detail by wrapping things in the detailed function which is equivalent to the --explain flag on the command line:

*Main Dhall> detailed (input auto "{ _1 = 1, _2 = 2, _3 = 3, _4 = 5 } : { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural}" :: IO IP)
*** Exception: 
Error: Expression doesn't match annotation

{ + _2 : …
, + _3 : …
, + _4 : …
,   _1 : - { … : … }
         + Natural
}

Explanation: You can annotate an expression with its type or kind using the     
❰:❱ symbol, like this:                                                          


    ┌───────┐                                                                   
    │ x : t │  ❰x❱ is an expression and ❰t❱ is the annotated type or kind of ❰x❱
    └───────┘                                                                   

The type checker verifies that the expression's type or kind matches the        
provided annotation                                                             

For example, all of the following are valid annotations that the type checker   
accepts:                                                                        


    ┌─────────────┐                                                             
    │ 1 : Natural │  ❰1❱ is an expression that has type ❰Natural❱, so the type  
    └─────────────┘  checker accepts the annotation                             


    ┌───────────────────────┐                                                   
    │ Natural/even 2 : Bool │  ❰Natural/even 2❱ has type ❰Bool❱, so the type    
    └───────────────────────┘  checker accepts the annotation                   


    ┌────────────────────┐                                                      
    │ List : Type → Type │  ❰List❱ is an expression that has kind ❰Type → Type❱,
    └────────────────────┘  so the type checker accepts the annotation          


    ┌──────────────────┐                                                        
    │ List Text : Type │  ❰List Text❱ is an expression that has kind ❰Type❱, so 
    └──────────────────┘  the type checker accepts the annotation               


However, the following annotations are not valid and the type checker will
reject them:                                                                    


    ┌──────────┐                                                                
    │ 1 : Text │  The type checker rejects this because ❰1❱ does not have type  
    └──────────┘  ❰Text❱                                                        


    ┌─────────────┐                                                             
    │ List : Type │  ❰List❱ does not have kind ❰Type❱                           
    └─────────────┘                                                             


Some common reasons why you might get this error:                               

● The Haskell Dhall interpreter implicitly inserts a top-level annotation       
  matching the expected type                                                    

  For example, if you run the following Haskell code:                           


    ┌───────────────────────────────┐                                           
    │ >>> input auto "1" :: IO Text │                                         
    └───────────────────────────────┘                                           


  ... then the interpreter will actually type check the following annotated     
  expression:                                                                   


    ┌──────────┐                                                                
    │ 1 : Text │                                                                
    └──────────┘                                                                


  ... and then type-checking will fail                                          

────────────────────────────────────────────────────────────────────────────────

You or the interpreter annotated this expression:                               

↳   { _1 = 1, _2 = 2, _3 = 3, _4 = 5 }
  : { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural }

... with this type or kind:                                                     

↳ { _1 : { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural } }

... but the inferred type or kind of the expression is actually:                

↳ { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural }

────────────────────────────────────────────────────────────────────────────────

{ _1 = 1, _2 = 2, _3 = 3, _4 = 5 } : { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural} : { _1 : { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural } }
(input):1:1

The key part is the bottom of the message, which says:

You or the interpreter annotated this expression:                               

↳   { _1 = 1, _2 = 2, _3 = 3, _4 = 5 }
  : { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural }

... with this type or kind:                                                     

↳ { _1 : { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural } }

... but the inferred type or kind of the expression is actually:                

↳ { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural }

... and that confirms that the extra 1-field record wrapping the type is what is interfering with decoding.

The reason for this unexpected type is because of how you derived the Interpret instance for IP here:

instance Interpret IP where

When you omit the Interpret instance implementation it falls back on using the Generic instance for IP which is NOT the same as the Generic instance for (Word8, Word8, Word8, Word8). You can confirm this by asking GHC to print out the generic representation of the two types:

*Main Dhall> import GHC.Generics
*Main Dhall GHC.Generics> :kind! Rep IP
Rep IP :: * -> *
= D1
    ('MetaData "IP" "Main" "main" 'True)
    (C1
       ('MetaCons "IP" 'PrefixI 'False)
       (S1
          ('MetaSel
             'Nothing 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy)
          (Rec0 (Word8, Word8, Word8, Word8))))
*Main Dhall GHC.Generics> :kind! Rep (Word8, Word8, Word8, Word8)
Rep (Word8, Word8, Word8, Word8) :: * -> *
= D1
    ('MetaData "(,,,)" "GHC.Tuple" "ghc-prim" 'False)
    (C1
       ('MetaCons "(,,,)" 'PrefixI 'False)
       ((S1
           ('MetaSel
              'Nothing 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy)
           (Rec0 Word8)
         :*: S1
               ('MetaSel
                  'Nothing 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy)
               (Rec0 Word8))
        :*: (S1
               ('MetaSel
                  'Nothing 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy)
               (Rec0 Word8)
             :*: S1
                   ('MetaSel
                      'Nothing 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy)
                   (Rec0 Word8))))

The Generic representation of the IP type is a record with one (anonymous) field, where that one field contains the 4-tuple of Word8s. The Generic representation of the (Word8, Word8, Word8, Word8) type is a record of 4 fields (each of which contains a Word8). You probably expected the latter behavior (an outermost record of 4 fields) rather than the former behavior (an outermost record of 1 field).

In fact, we can get the behavior you expected by decoding straight into a (Word8, Word8, Word8, Word8) type:

*Main Dhall GHC.Generics> detailed (input auto "{ _1 = 1, _2 = 2, _3 = 3, _4 = 5 } : { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural}" :: IO (Word8, Word8, Word8, Word8))
(1,2,3,5)

... although that doesn't really solve your problem :)

So if you want the IP type to have the same Interpret instance as (Word8, Word8, Word8, Word8) then you actually do not want to use GHC Generics to derive the Interpret instance for IP. What you actually want is to use GeneralizedNewtypeDeriving so that the newtype uses the exact same instance as the underlying type. You can do that with the following code:

{-# LANGUAGE DeriveGeneric              #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE RecordWildCards            #-}

import Control.Applicative  ( empty, pure )
import Dhall  ( Generic, Interpret( autoWith ), Type( Type, extract, expected ) )
import Dhall.Core  ( Expr( Natural, NaturalLit ) )
import Data.Word  ( Word8 )

newtype IP = IP (Word8, Word8, Word8, Word8)
  deriving (Interpret, Show)

word8 :: Type Word8
word8 = Type {..}
  where
    extract (NaturalLit n) | n >= 0 && n <= 255 = pure (fromIntegral n)
    extract  _             = empty

    expected = Natural

instance Interpret Word8 where
  autoWith _ = word8

instance (Interpret a,Interpret b,Interpret c,Interpret d) => Interpret (a,b,c,d)

The main changes I made were:

• Adding the GeneralizedNewtypeDeriving language extension
• Removing the Generic instance for IP
• Adding a Show instance for IP (for debugging)

... and then that works:

*Main Dhall GHC.Generics> input auto "{ _1 = 1, _2 = 2, _3 = 3, _4 = 5 } : { _1 : Natural, _2 : Natural, _3 : Natural, _4 : Natural}" :: IO IP
IP (1,2,3,5)

You can also do this without any orphan instances, like this:

{-# LANGUAGE RecordWildCards #-}

import Control.Applicative (empty, pure)
import Data.Coerce (coerce)
import Dhall (Interpret(..), Type(..), genericAuto)
import Dhall.Core (Expr(..))
import Data.Word (Word8)

newtype MyWord8 = MyWord8 Word8

word8 :: Type MyWord8
word8 = Type {..}
  where
    extract (NaturalLit n)
        | n >= 0 && n <= 255 = pure (MyWord8 (fromIntegral n))
    extract _ =
        empty

    expected = Natural

instance Interpret MyWord8 where
  autoWith _ = word8

newtype IP = IP (Word8, Word8, Word8, Word8)
    deriving (Show)

instance Interpret IP where
    autoWith _ = coerce (genericAuto :: Type (MyWord8, MyWord8, MyWord8, MyWord8))