[Haskell-cafe] Constructors on left and right hand sides of equation

[PATRICK BROWNE]

On 19 September 2014 06:05, Richard A. O'Keefe <ok at cs.otago.ac.nz> wrote:

> 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)
>
> First, (Fill m i) isn't *any* kind of constructor.
> Second, the first occurrence of (Fill m i) is a (data) pattern
> containing the (data) constructor Fill, and the second occurrence
> of (Fill m i) is an expression containing the (data) constructor
> Fill.  We may not know what Glass, NewGlass, and Fill *mean*,
> but we know what they *are* (as Haskell thingies).



Your response has clarified most of my confusion.
Please bear with my naivety  as seek to ride my mind of confusion over
these two point:

1) I do not know what the phrase "(Fill m i) isn't *any* kind of
constructor." means.
2) Does "Fill (NewGlass (size m))  ((size m) - j) " create two Glass
Haskell thingies?

The context of this code is an attempt to formalize the drinking
operation[1].
[1] www.ncgia.ucsb.edu/Publications/Tech_Reports/95/95-7.pdf

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