PATRICK BROWNE
drink (NewGlass i) j = (NewGlass i) -- Eq 1
In Eq 1 is (NewGlass i) on the LHS distinct from (NewGlass i) on the RHS?
drink (Fill m i) j | full (Fill m i) = Fill (NewGlass (size m)) ((size m) - j) -- Eq2
In Eq2 are the occurrences of the data constructor Fill on the LHS and RHS distinct?
Yes they're distinct. This is important since their types can be different. Here's an extreme example:
data Proxy a = Proxy
foo :: Proxy Int -> Proxy Bool foo Proxy = Proxy
It looks like "foo" is the identity function, returning its argument value, similar to these "bar" functions:
bar1 :: () -> () bar1 () = ()
bar2 :: () -> () bar2 = id
However, it's not:
foo2 :: Proxy Int -> Proxy Bool foo2 = id
[1 of 1] Compiling Main ( test.hs, interpreted )
test.hs:7:8: Couldn't match type `Int' with `Bool' Expected type: Proxy Int -> Proxy Bool Actual type: Proxy Int -> Proxy Int In the expression: id In an equation for `foo2': foo2 = id Failed, modules loaded: none.
Hence, in general, we must assume that they're different values. We tend to use different terminology too, for example "patterns" on the left, "expressions" on the right; "destructing" on the left, "constructing" on the right. Cheers, Chris