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Today's Topics:
1. "Phantom" function? (Baa)
2. Re: "Phantom" function? (Baa)
3. Re: "Phantom" function? (Baa)
----------------------------------------------------------------------
Message: 1
Date: Fri, 15 Sep 2017 12:40:46 +0300
From: Baa <[email protected]>
To: "The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell" <[email protected]>
Subject: [Haskell-beginners] "Phantom" function?
Message-ID: <20170915124046.286a8049@Pavel>
Content-Type: text/plain; charset=UTF-8
Hello, List. I'm trying to write function like this:
type VerifyJson a = IO Bool
isRight :: FromJSON a => FilePath -> VerifyJson a
isRight testbed = do
js <- readFile testbed
return $ isJust (decode js) <<<<< ERROR IS HERE !
So I get error:
60 23 error error:
• Could not deduce (FromJSON a0) arising from a use of ‘decode’
from the context: FromJSON a
bound by the type signature for:
isRight :: FromJSON a => FilePath -> VerifyJson a
at
/home/XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX/.stack-work/intero/intero7476qP4.hs:57:1-46
The type variable ‘a0’ is ambiguous
These potential instances exist:
instance FromJSON DotNetTime
-- Defined in ‘aeson-1.0.2.1:Data.Aeson.Types.FromJSON’
instance FromJSON Value
-- Defined in ‘aeson-1.0.2.1:Data.Aeson.Types.FromJSON’
instance (FromJSON a, FromJSON b) => FromJSON (Either a b)
-- Defined in ‘aeson-1.0.2.1:Data.Aeson.Types.FromJSON’
...plus 25 others
...plus 52 instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
• In the first argument of ‘MB.isJust’, namely ‘(decode js)’
In the second argument of ‘($)’, namely ‘MB.isJust (decode js)’
In a stmt of a 'do' block: return $ MB.isJust (decode js) (intero)
Is it possible to write such (phantom?;)) function? Sure, I can make workaround
like:
class FromJSON a => VerifyJson a where
isRight :: FilePath -> IO Bool
isRight testbed = do
js <- readFile testbed
return $ MB.isJust (decode js::Maybe a)
but in this case I must declare instances for all verificating types (right?),
like:
instance VerifyJson MyType
But idea was to minimize `isRight` users code...
===
Best regards, Paul
------------------------------
Message: 2
Date: Fri, 15 Sep 2017 12:53:46 +0300
From: Baa <[email protected]>
To: "The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell" <[email protected]>
Subject: Re: [Haskell-beginners] "Phantom" function?
Message-ID: <20170915125346.212bffc6@Pavel>
Content-Type: text/plain; charset=UTF-8
Seems that this works:
{-# LANGUAGE ExistentialQuantification #-}
...
type VerifyJson a = IO Bool
isRight :: forall a. FromJSON a => FilePath -> VerifyJson a
isRight testbed = do
js <- readFile testbed
return $ MB.isJust (decode js::Maybe a)
but what is the difference?!?
> Hello, List. I'm trying to write function like this:
>
> type VerifyJson a = IO Bool
>
> isRight :: FromJSON a => FilePath -> VerifyJson a
> isRight testbed = do
> js <- readFile testbed
> return $ isJust (decode js) <<<<< ERROR IS HERE !
>
> So I get error:
>
> 60 23 error error:
> • Could not deduce (FromJSON a0) arising from a use of ‘decode’
> from the context: FromJSON a
> bound by the type signature for:
> isRight :: FromJSON a => FilePath -> VerifyJson a
> at
> /home/XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX/.stack-work/intero/intero7476qP4.hs:57:1-46
> The type variable ‘a0’ is ambiguous
> These potential instances exist:
> instance FromJSON DotNetTime
> -- Defined in ‘aeson-1.0.2.1:Data.Aeson.Types.FromJSON’
> instance FromJSON Value
> -- Defined in ‘aeson-1.0.2.1:Data.Aeson.Types.FromJSON’
> instance (FromJSON a, FromJSON b) => FromJSON (Either a b)
> -- Defined in ‘aeson-1.0.2.1:Data.Aeson.Types.FromJSON’
> ...plus 25 others
> ...plus 52 instances involving out-of-scope types
> (use -fprint-potential-instances to see them all)
> • In the first argument of ‘MB.isJust’, namely ‘(decode js)’
> In the second argument of ‘($)’, namely ‘MB.isJust (decode js)’
> In a stmt of a 'do' block: return $ MB.isJust (decode js)
> (intero)
>
> Is it possible to write such (phantom?;)) function? Sure, I can make
> workaround like:
>
> class FromJSON a => VerifyJson a where
> isRight :: FilePath -> IO Bool
> isRight testbed = do
> js <- readFile testbed
> return $ MB.isJust (decode js::Maybe a)
>
> but in this case I must declare instances for all verificating types
> (right?), like:
>
> instance VerifyJson MyType
>
> But idea was to minimize `isRight` users code...
>
> ===
> Best regards, Paul
>
------------------------------
Message: 3
Date: Fri, 15 Sep 2017 13:05:52 +0300
From: Baa <[email protected]>
To: "The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell" <[email protected]>
Subject: Re: [Haskell-beginners] "Phantom" function?
Message-ID: <20170915130552.391143f6@Pavel>
Content-Type: text/plain; charset=UTF-8
unfortunately call of new version:
..
(isRight "data/a.json" :: VerifyJson RESTResp) `shouldReturn` True
..
leads to new errors:
• Ambiguous type variable ‘a0’ arising from a use of ‘isRight’
prevents the constraint
‘(aeson-1.0.2.1:Data.Aeson.Types.FromJSON.FromJSON
a0)’ from being solved.
Probable fix: use a type annotation to specify what ‘a0’ should be.
These potential instances exist:
.....
• In the first argument of ‘shouldReturn’, namely
‘(isRight "data/a.json" :: VerifyJson RESTResp)’
In a stmt of a 'do' block:
(isRight "data/a.json" :: VerifyJson RESTResp) `shouldReturn` True
In the second argument of ‘($)’, namely
‘do { (isRight "data/a.json" :: VerifyJson RESTResp)
`shouldReturn` True }’
I will be grateful for any help.
===
Best regards, Paul
> Seems that this works:
>
> {-# LANGUAGE ExistentialQuantification #-}
> ...
>
> type VerifyJson a = IO Bool
>
> isRight :: forall a. FromJSON a => FilePath -> VerifyJson a
> isRight testbed = do
> js <- readFile testbed
> return $ MB.isJust (decode js::Maybe a)
>
> but what is the difference?!?
>
>
> > Hello, List. I'm trying to write function like this:
> >
> > type VerifyJson a = IO Bool
> >
> > isRight :: FromJSON a => FilePath -> VerifyJson a
> > isRight testbed = do
> > js <- readFile testbed
> > return $ isJust (decode js) <<<<< ERROR IS HERE !
> >
> > So I get error:
> >
> > 60 23 error error:
> > • Could not deduce (FromJSON a0) arising from a use of ‘decode’
> > from the context: FromJSON a
> > bound by the type signature for:
> > isRight :: FromJSON a => FilePath -> VerifyJson
> > a
> > at
> > /home/XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX/.stack-work/intero/intero7476qP4.hs:57:1-46
> > The type variable ‘a0’ is ambiguous These potential instances exist:
> > instance FromJSON DotNetTime
> > -- Defined in ‘aeson-1.0.2.1:Data.Aeson.Types.FromJSON’
> > instance FromJSON Value
> > -- Defined in ‘aeson-1.0.2.1:Data.Aeson.Types.FromJSON’
> > instance (FromJSON a, FromJSON b) => FromJSON (Either a b)
> > -- Defined in ‘aeson-1.0.2.1:Data.Aeson.Types.FromJSON’
> > ...plus 25 others
> > ...plus 52 instances involving out-of-scope types
> > (use -fprint-potential-instances to see them all)
> > • In the first argument of ‘MB.isJust’, namely ‘(decode js)’
> > In the second argument of ‘($)’, namely ‘MB.isJust (decode
> > js)’ In a stmt of a 'do' block: return $ MB.isJust (decode js)
> > (intero)
> >
> > Is it possible to write such (phantom?;)) function? Sure, I can make
> > workaround like:
> >
> > class FromJSON a => VerifyJson a where
> > isRight :: FilePath -> IO Bool
> > isRight testbed = do
> > js <- readFile testbed
> > return $ MB.isJust (decode js::Maybe a)
> >
> > but in this case I must declare instances for all verificating types
> > (right?), like:
> >
> > instance VerifyJson MyType
> >
> > But idea was to minimize `isRight` users code...
> >
> > ===
> > Best regards, Paul
> >
>
------------------------------
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