[Haskell-cafe] Record syntax, reopening a can of worms.

[Yves Parès]

> case myData of
>   myA at A{} -> fooForA's myA
>   myB at B{} -> fooForB's myB

I think this would typecheck if you used GADTs.
Actually what you'd want is to use the record syntax with GADTs (there are
to add the extra type safety you want), however both are not compatible.

data ALike
data BLike

data MyData x where
    A :: Int -> Int -> MyData ALike
    B :: Int -> MyData BLike

a (A x _) = x
b (A _ x) = x
c (B x) = x
-- GHC may require you to write the type signatures for those three
functions.
Le 27 mai 2012 10:52, <timothyhobbs at seznam.cz> a écrit :

> Your Maybe example is very contrived.  The place where I ran into this was
> much less contrived I think.
>
> I have an editor for a visual programming language. That looks like this:
>
> https://github.com/timthelion/gridhaskell-haskarrow/wiki
>
> I'm using a modified version of the Document-View model for application
> design.  The commands in the language are defined in the Cell data type:
>
>
> https://github.com/timthelion/gridhaskell-haskarrow/blob/master/Cell.lhs
>
>
> Some Cell types, like Jump, have a long crooked line comming out of them
> called a Path.  This line is held in the path :: Maybe Path feild.
>
>
> When I'm drawing the Cell tree, I have a function that extracts these
> Paths.
> https://github.com/timthelion/gridhaskell-haskarrow/blob/master/ArrowDrawing.lhs#L75
>
>
> It used to be, that more types of Cell than just the Jump Cell had a path.
>
>
> As I removed these Paths from the Cell datatype, my line drawing function
> started crashing, whenever it encountered those Cell types.  By having
> chosen to use the short hand record selector syntax rather than the long
> hand place value syntax, I caused a runtime error in my code.
>
>
> I could of course have written the same function like this:
>
> >   where path_points =
> >          case
> >           case cell of
>
> Types of cells which have paths:
>
> >             Cell.Jump _ path        -> Just path
> >             _                  -> Nothing of
> >          Just path -> maybe [] (\p -> [Path.point p]) (Cell.path cell)
> >          Nothing   -> []
>
>
> Record selection is a more convenient syntax.  It *could* be usefull for
> making more maintainable code(the reason why I choose it).  The method I
> have chosen *would* be more maintainable in the case I want to add another
> feild to Jump.  In that case I would not have to re-write the path_points
> function.  Sadly, due to GHC's unnessesary and complete inability to do
> type safety checking, what should have been a convenient tool, has become
> an unnessecary source of runtime errors!
>
>
> How would I use your syntax in functions that want to pattern match
> against both A and B?  I tried this without success:
>
> >data ALike
> >data BLike
>
> >data MyData t = A {a::Int,
> > b::Int} |
> > B {c::Int}
>
> >mkA x y = A x y :: MyData ALike
> >mkB x = B x :: MyData BLike
>
> altRecordSyntaxes.lhs:15:18:
>     Couldn't match type `BLike' with `ALike'
>     Expected type: MyData ALike
>       Actual type: MyData t
>     In the first argument of `g', namely `myA'
>     In the expression: g myA
>
> >foo :: MyData t -> Int
>
> >foo myA at A{} = g myA
>
> > where
>
> >  g :: MyData ALike -> Int
>
> >  g myA' = a myA'
>
> >foo myB at B{} = g myB
>
> > where
>
> >  g :: MyData BLike -> Int
>
> >  g myB' = c myB'
>
> >main :: IO ()
>
> >main = do
>
> >  print $ foo $ mkA 1 3
>
>
> This also doesn't work:
>
> >foo :: MyData t -> Int
>
> >foo myData =
>
> > case myData of
>
> >  myA at A{} -> fooForA's myA
>
> >  myB at B{} -> fooForB's myB
>
> > where
>
> >  fooForA's :: MyData ALike -> Int
>
> >  fooForA's myA' = a myA'
>
> >  fooForB's :: MyData BLike -> Int
>
> >  fooForB's myB' = a myB'
>
> >main :: IO ()
>
> >main = do
>
> >  print $ foo $ mkA 1 3
>
> My solution looks very clean(except for that nasty looking data
> declaration) and has the same advantages.  Really, the current record
> syntax is just flawed :D
>
> >data MyData = A A' | B B'
>
> >data A' = A'{a::Int,
> >             b::Int}
> >data B' = B'{c::Int}
>
> >foo :: MyData -> Int
> >foo (A myA) = a myA
> >foo (B myB) = c myB
>
> >main :: IO ()
> >main = print $ foo (A (A' 1 2))
>
> Timothy
>
> ---------- Původní zpráva ----------
> Od: John Meacham <john at repetae.net>
> Datum: 27. 5. 2012
> Předmět: Re: [Haskell-cafe] Record syntax, reopening a can of worms.
>
> Is it any more ridiculous than
>
> > f x at Nothing {} = fromJust x
> > main = print (f Nothing)
>
> crashing at run time? That is what you are expressing with your first
> one. This issue is completely unrelated to the named field syntax,
> they behave exactly like data types with non-named fields.
>
> However, you can achieve something like what you want with phantom types.
>
> > data ALike
> > data BLike
>
> >data MyData t = A {a::Int,
> > b::Int} |
> > B {c::Int}
>
> > mkA x y = A x y :: MyData ALike
> > mkB x = B x :: MyData BLike
>
> then you can write functions of
> 'MyData ALike' to indicate it will only have 'A' as a constructor
> 'MyData BLike' to indicate it will only have 'B'
> and 'forall t . MyData t' for functions that can take a general MyData
> that can have either.
>
> John
>
>
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