[Haskell-cafe] Replace data constructors via meta programming

David Fox dsf at seereason.com
Thu Feb 15 20:00:44 UTC 2018


True, but my point about pattern matching constructors stands.‚Äč

On Thu, Feb 15, 2018 at 6:17 AM, Vilem-Benjamin Liepelt <vl81 at kent.ac.uk>
wrote:

> PS: I meant cases `Var {}` and `Not {}`.
>
> On 2018-02-15, at 14:16, Vilem-Benjamin Liepelt <vl81 at kent.ac.uk> wrote:
>
> Hi David,
>
> I don't think this is well-typed. GHC seems to infer `Proposition a ->
> Bool -> Bool` (by majority vote?) but obviously then complains about the
> cases `id` and `not`.
>
> I believe that there is a way to do this with dependent types, but not
> sure whether this is possible in Haskell.
>
> Best,
>
> Vilem
>
> On 2018-02-15, at 13:51, David Fox <dsf at seereason.com> wrote:
>
> You actually can pattern match on constructor only:
>
> magic = \case
>     Var {} -> id
>     Not  {}-> not
>     And {} -> (&&)
>     Or {} -> (||)
>     If {} -> (==>)
>     Iff {} -> (==)
>
> On Sun, Feb 11, 2018 at 6:30 PM, Vilem-Benjamin Liepelt <vl81 at kent.ac.uk>
> wrote:
>
>> Hi,
>>
>> I am looking for a solution to get rid of this silly boilerplate:
>>
>> eval :: Ord var => Map var Bool -> Proposition var -> Bool
>> eval ctx prop = evalP $ fmap (ctx Map.!) prop
>>   where
>>     evalP = \case
>>         Var b -> b
>>         Not q -> not $ evalP q
>>         And p q -> evalP p && evalP q
>>         Or p q -> evalP p || evalP q
>>         If p q -> evalP p ==> evalP q
>>         Iff p q -> evalP p == evalP q
>>
>> What I would like to do in essence is to replace the data constructors
>> like so:
>>
>> -- Not valid Haskell!! Can't pattern match on constructor only...
>> magic = \case
>>     Var -> id
>>     Not -> not
>>     And -> (&&)
>>     Or -> (||)
>>     If -> (==>)
>>     Iff -> (==)
>>
>> compile = transformAST magic $ fmap (\case 'P' -> False; 'Q' -> True)
>>
>> >>> compile (Iff (Not (And (Var 'P') (Var 'Q'))) (Or (Not (Var 'P')) (Not
>> (Var 'Q'))))
>>             ((==) (not ((&&) (id True) (id False))) ((||) (not (id True))
>> (not (id False))))
>>
>> Note how the compiled expression exactly mirrors the AST, so there should
>> be some meta programming technique for this.
>>
>> Does anyone have an idea how I can achieve this?
>>
>> The full source code is here: https://gist.github.com/vimuel
>> /7dcb8a9f1d2b7b72f020d66ec4157d7b
>>
>> I am happy to take any other comments relating to my code...
>>
>> Best,
>>
>> Vilem
>>
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>
>
>
>
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