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Safe pattern matching

Suppose we have the following variants:

x,y,z :: V '[String,Int,Float]
x = V "test"
y = V @Int 10
z = V @Float 5.0

Unsafe pattern-matching

We can use pattern matching on the constructors:

printV' :: V '[String,Int,Float] -> IO ()
printV' = \case
   V (s :: String) -> putStrLn ("Found string: " ++ s)
   V (i :: Int)    -> putStrLn ("Found int: " ++ show i)
   V (f :: Float)  -> putStrLn ("Found float: " ++ show f)
   _               -> undefined

> printV' x
Found string: test
> printV' y
Found int: 10
> printV' z
Found float: 5.0

However the compiler cannot detect that the pattern matching is complete, hence we have the choice between a warning or adding a wildcard match as we have done above.

Safe pattern-matching with continuations

Another solution is to rely on multi-continuations. Then we can provide a function per constructor as in a pattern-matching. For instance, with multi-continuations we can transform a variant V '[A,B,C] into a function whose type is (A -> r, B -> r, C -> r) -> r. Hence the compiler will ensure that we provide the correct number of alternatives in the continuation tuple (the first parameter).

Transforming a Variant into a multi-continuation is done with toCont. Mapping the continuation tuple is done with >::>.

import Haskus.Utils.ContFlow

printV :: V '[String,Int,Float] -> IO ()
printV v = toCont v >::>
   ( \s -> putStrLn ("Found string: " ++ s)
   , \i -> putStrLn ("Found int: " ++ show i)
   , \f -> putStrLn ("Found float: " ++ show f)

Unordered continuations (>:%:>)

By using the >:%:> operator instead of >::>, we can provide continuations in any order as long as an alternative for each constructor is provided.

The types must be unambiguous as the Variant constructor types can't be used to infer the continuation types (as is done with >::>). Hence the type ascriptions in the following example:

printU :: V '[String,Int,Float] -> IO ()
printU v = toCont v >:%:>
   ( \f -> putStrLn ("Found float: " ++ show (f :: Float))
   , \s -> putStrLn ("Found string: " ++ s)
   , \i -> putStrLn ("Found int: " ++ show (i :: Int))

Splitting constructors

We can chose to handle only a subset of the values of a Variant by using splitVariant.

For instance in the following example we only handle Int and Float values. The other ones are considered as left-overs:

printNum v = case splitVariant @'[Float,Int] v of
   Right v -> toCont v >:%:>
      ( \f -> putStrLn ("Found float: " ++ show (f :: Float))
      , \i -> putStrLn ("Found int: " ++ show (i :: Int))
   Left leftovers -> putStrLn "Not a supported number!"

> printNum x
Not a supported number!
> printNum y
Found int: 10
> printNum z
Found float: 5.0

The code is generic and can be used with any Variant type:

w,k,u :: V '[String,Int,Double,Maybe Int]
w = V @Double 1.0
k = V (Just @Int 10)
u = V @Int 17

> printNum w
Not a supported number!
> printNum k
Not a supported number!
> printNum u
Found int: 17