[Haskell-cafe] Proposal: Sum type branches as extended types (as
griba2001 at gmail.com
Thu Jun 3 11:14:22 EDT 2010
Extending sum types with data constructors would spare runtime errors or
when applying functions to inappropriate branches, as in the example ...
data List a = Nil | Cons a (List a) -- List!Nil and List!Cons
-- as extended types
* Actual system, with runtime errors (as in GHC Data.List head) or
hd :: List a -> a
hd (Cons x _) -> x
hd Nil -> error "error: hd: empty list" -- error or exception throwing
* Proposed system extending types with constructors as Type!Constructor:
User must do pattern matching before applying the constructor-specific
In ''var @ (Constructor _ _)'' the compiler should append the constructor
to the type as a pair (Type, Constructor) as an extended type for ''var''
No need for runtime errors or exception control
hd :: List!Cons a -> a
hd (Cons x _) = x
headOf :: List a -> Maybe a
headOf list = case list of
li @ (Cons _ _) -> Just hd li -- extTypeOf li == ( 'List', 'Cons')
-- should pass typechecker for List!Cons
li @ Nil -> Just hd li -- compiler error !!
-- extTypeOf ('List','Nil') don't match
_ -> Just hd list -- compiler error !!
-- extTypeOf ('List',Nothing) don't match
Maybe we could take out importance on the number of _ wildcards (constructor
arity) with a syntax like.
li @ (Cons ...)
li @ (Nil ...)
Gabriel Riba Faura.
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