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

27 May 2012


      ...
case myData of
  myA@A{} -> fooForA's myA
  myB@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,  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...
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@A{} = g myA
...
where
...
g :: MyData ALike -> Int
...
g myA' = a myA'
...
foo myB@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@A{} -> fooForA's myA
...
myB@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 
Datum: 27. 5. 2012
Předmět: Re: [Haskell-cafe] Record syntax, reopening a can of worms.
Is it any more ridiculous than
...
f x@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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Re: [Haskell-cafe] Record syntax, reopening a can of worms.

Yves Parès