[Haskell-beginners] phantom types and record syntax

[Dimitri DeFigueiredo]

Thanks for the thoughts.

This is funny, so the constructor has the same name but is for different 
types. I think this is specially bad for security. I'm going to be 
paranoid about it now.

Dimitri


On 17/06/15 00:42, Karl Voelker wrote:
> On Tue, Jun 16, 2015, at 05:43 PM, Dimitri DeFigueiredo wrote:
>> I am a little suprised that this program compiles in GHC:
>>
>> ----
>> data ReqTime = ReqTime
>> data AckTime = AckTime
>>
>> data Order a = Order { price  ::Double, volume ::Int, timestamp ::Int }
>>
>> convertToReq :: Order AckTime -> Order ReqTime
>> convertToReq o = o{price = 1}
>>
>> main = putStrLn "Hi!"
>> ----
> I found it surprising, too. But, if you look in the Haskell report
> (https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-540003.15.3),
> record update is defined by a "desugaring" translation. So your
> convertToReq desugars (roughly) to:
>
> convertToReq o = case o of
>      Order v1 v2 v3 -> Order 1 v2 v3
>
> Unfortunately, the report does not have any commentary on why it is the
> way it is.
>
> -Karl
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