On Thu, Sep 18, 2014 at 4:08 AM, Richard A. O'Keefe
On 18/09/2014, at 2:03 AM, PATRICK BROWNE wrote:
Dear list In Eq 1 is (NewGlass i) on the LHS distinct from (NewGlass i) on the RHS? In Eq2 are the occurrences of the data constructor Fill on the LHS and RHS distinct?
data Glass = NewGlass Int | Fill Glass Int deriving Show drink :: Glass -> Int -> Glass drink (NewGlass i) j = (NewGlass i) -- Eq 1 drink (Fill m i) j | full (Fill m i) = Fill (NewGlass (size m)) ((size m) - j) -- Eq 2 | (i >= j) = Fill m (i-j) | otherwise = drink m (j-1)
The question question is "what do you MEAN by 'are they distinct'"?
They are two distinct references to a single binding occurrence of an identifier. In Eq1, the first occurrence is in a pattern, and the second is in an expression.
I wonder if you were interested in the question whether the second occurrence of NewGlass i would allocate new storage or whether it would use whatever the first occurrence matched? In that case, I believe the answer is "it's up to the compiler". You can *ask* for the matched data to be shared:
drink g@(NewGlass _) _ = g drink g@(Fill m i ) j | full g = Fill (NewGlass (size m)) (size m - j) | i >= j = Fill m (i - j) | True = drink m (j - 1)
Someone else has already pointed out
data Proxy a = Proxy
-- foo :: Proxy a -> Proxy b foo Proxy = Proxy -- Eq3
where the two occurrences of Proxy in Eq3 can have different types. But note that
-- bar :: Proxy a -> Proxy a bar p@Proxy = p -- Eq4
The two occurrences of the variable p in Eq4 must have the same type (and the same value).
Since we're talking about allocating new storage: even though 'Eq3' has a second occurrence of 'Proxy' on the right hand side, I believe in GHC's implementation these are shared and there is only one Proxy at runtime. Of course this is implementation dependent. Regards, Erik