Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>Here's one way that kind of works. There are many more cases to cover if your fA fB have type variables in their inferred types. In that case the following code will have some pattern match failures at compile time.</p> <pre><code>{-# LANGUAGE FlexibleInstances, FunctionalDependencies, TemplateHaskell #-} import Language.Haskell.TH data A = A{ax::Int} deriving(Show) data B = B{bx::Int} deriving(Show) fA A{} i = i*(2 :: Int) fB B{} s1 s2 = "test"++s1++s2 class Func_f a b | a -&gt; b where f :: a -&gt; b let getLetter (AppT (AppT _ x) _) = x getSnd (AppT x y) = y mkInst name0 = do (VarI n ty _ _) &lt;- reify name0 fmap (:[]) $ instanceD (return []) [t| Func_f $(return $ getLetter ty) $(return $ getSnd ty) |] [valD (varP 'f) (normalB (varE name0)) []] in fmap concat $ mapM mkInst ['fB, 'fA] main :: IO () main = do let a = A 1 b = B 2 a_out = f a 5 b_out = f b "a" "b" print a_out print b_out </code></pre> <p>Whether this even helps you with the language you're compiling to haskell is another question.</p> <hr> <p><strong>Added Hints</strong></p> <p>If the type is polymorphic, you'll see reify giving something that isn't covered by my example code above.</p> <pre><code> &gt; :set -XTemplateHaskell &gt; :m +IPPrint Language.Haskell.TH &gt; putStrLn $(reify 'id &gt;&gt;= stringE . pshow) </code></pre> <p>Prints out something that describes (a->a):</p> <pre><code>VarI GHC.Base.id (ForallT [PlainTV a_1627394484] [] (AppT (AppT ArrowT (VarT a_1627394484)) (VarT a_1627394484))) Nothing (Fixity 9 InfixL) </code></pre> <p>With a bit of work, you could split that Type in there into the CxtQ and TypeQ that instanceD needs.</p> <p>I don't know how much sense it makes to generate (a->b) instead. You could go about replacing all type variables with new unique ones, which is probably best done with something like Data.Generics.everywhereM because data Type has many many constructors.</p> <p>You'll still have an issue that this is invalid:</p> <pre><code>instance Func_f (a -&gt; b) where f _ = id </code></pre> <p>This approach to getting (a->b) can make your program crash:</p> <pre><code>instance Func_f (a -&gt; b) where f _ = unsafeCoerce id </code></pre> <p>This approach lets the instance get chosen when the types are different, but at some later point in ghc's execution you can end up with a failure if 'a' and 'b' can't be the same.</p> <pre><code>instance (a~b) =&gt; Func_f (a-&gt;b) where f _ = unsafeCoerce id </code></pre>
    singulars
    1. This table or related slice is empty.
    1. POType inference of instance functions
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USaavogt
    1. USaavogt
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POType inference of instance functions
      singulars
      1. PTQuestion
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COthank you for this example - it works very well. Could you please tell me if is it possible to somehow use this technique to also support functions, which are not strictly typed, like `fA A{} i = i` ?
      singulars
      1. PO
      1. USWojciech Danilo
    2. COI've edited the question - added `C` data type and related instance to better show the problem. If it would be possible to use you roslution with such functions like `fA A{} i = i` it would be a lifesaver for me :)
      singulars
      1. PO
      1. USWojciech Danilo
    3. COI've added some more hints for getting that C (or your original A) accepted: template haskell is not known for being pleasant, so I've left you with the exercise of writing that code ;)
      singulars
      1. PO
      1. USaavogt
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload