Re: [Haskell-cafe] an idea for modifiyng data/newtype syntax: use `::=` instead of `=`

8 Aug 2015


      On Sat, Aug 08, 2015 at 11:18:18PM +0200, MigMit wrote:
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On 08 Aug 2015, at 23:16, Tom Ellis  wrote:
On Sat, Aug 08, 2015 at 11:14:40PM +0200, MigMit wrote:
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On 08 Aug 2015, at 23:11, Tom Ellis  wrote:
On Sat, Aug 08, 2015 at 11:02:27PM +0200, MigMit wrote:
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What you say is "if we don't use some perfectly legitimate language
constructs and never use third-party code, than 'data' and 'newtype' are
the same".
No, not at all.  I'm making a much stronger claim than that.  I'm claiming
the functionality provided by newtype is completely subsumed by that
provided by data.
Well, it's certainly unsupported by evidence. Because third-party code CAN
distinguish between those.
OK, show me the code!
I did. You removed it when quoting.
Reinstating:
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Prelude> newtype A = A Int deriving Show                                                                                           
Prelude> data B = B !Int deriving Show                                                                                             
Prelude> let x = case x of A n -> A 1 in x                                                                                         
A 1                                                                                                                                
Prelude> let y = case y of B n -> B 1 in y                                                                                         
*** Exception: <<loop>>
This isn't third party code.  It knows about the constructors of A and B.