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Today's Topics:
1. Re: phantom types and record syntax (Karl Voelker)
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Message: 1
Date: Tue, 16 Jun 2015 23:42:06 -0700
From: Karl Voelker <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] phantom types and record syntax
Message-ID:
<1434523326.2978810.297687737.0022a...@webmail.messagingengine.com>
Content-Type: text/plain
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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