Pattern-matching syntax not Constructor application syntax

user_2840542 0
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Given a two-place data constructor, I can partially apply it to one argument then apply that to the second. Why can't I use the same syntax for pattern matching?

data Point = MkPoint Float Float

x = 1.0 :: Float; y = 2.0 :: Float

thisPoint = ((MkPoint x) y)          -- partially apply the constructor

(MkPoint x1 y1) = thisPoint          -- pattern match OK

((MkPoint x2) y2) = thisPoint        -- 'partially apply' the pattern, but rejected: parse error at y2

((MkPoint x3 y3)) = thisPoint        -- this accepted, with double-parens

Why do I want to do that? I want to grab the constructor and first arg as an as-pattern, so I can apply it to a different second arg. (Yes the work-round in this example is easy. Realistically I have a much more complex pattern, with several args of which I want to split out the last.):

(mkPx@(MkPoint x4) y4) = thisPoint   -- also parse error

thatPoint = mkPx (y4 * 2)
pattern-matching haskell
2 Answers

I think there's no fundamental reason to prevent this kind of match.

Certainly it wouldn't do to allow you to write

f (MkPoint x1) = x1

and have that match a partially-applied constructor, i.e. a function. So, one reason to specify it as it was specified here is for simplicity: the RHS of an @ has to be a pattern. Simple, easy to parse, easy to understand. (Remember, the very first origins of the language were to serve as a testbed for PL researchers to tinker. Simple and uniform is the word of the day for that purpose.) MkPoint x1 isn't a pattern, therefore mkPx@(MkPoint x1) isn't allowed.

I suspect that if you did the work needed to carefully specify what is and isn't allowed, wrote up a proposal, and volunteered to hack on the parser and desugarer as needed, the GHC folks would be amenable to adding a language extension. Seems like a lot of work for not much benefit, though.

Perhaps record update syntax will scratch your itch with much less effort.

data Point = MkPoint {x, y :: Float}
m@(MkPoint { x = x5 }) = m { x = x5 + 1 }

You also indicate that, aside from the motivation, you wonder what part of the Report says that the pattern you want can't happen. The relevant grammar productions from the Report are here:

pat   →   lpat

lpat  →   apat
      |   gcon apat1 … apatk      (arity gcon  =  k, k ≥ 1)

apat  →   var [ @ apat]           (as pattern)
      |   gcon                    (arity gcon  =  0)
      |   ( pat )                 (parenthesized pattern)

(I have elided some productions that don't really change any of the following discussion.)

Notice that as-patterns must have an apat on their right-hand side. apats are 0-arity constructors (in which case it's not possible to partially apply it) or parenthesized lpats. The lpat production shown above indicates that for constructors of arity k, there must be exactly k apat fields. Since MkPoint has arity 2, MkPoint x is therefore not an lpat, and so (MkPoint x) is not an apat, and so m@(MkPoint x) is not an apat (and so not produced by pat → lpat → apat).

user_7916040

I can partially apply [a constructor] to one argument then apply that to the second.

   thisPoint = ((MkPoint x) y)          -- partially apply the constructor

There's another way to achieve that without parens, also I can permute the arguments

thisPoint = MkPoint x $ y
thisPoint = flip MkPoint y $ x

Do I expect I could pattern match on that? No, because flip, ($) are just arbitrary functions/operators.

I want to grab the constructor and first arg as an as-pattern, ...

What's special about the first arg? Or the all-but-last arg (since you indicate your real application is more complex)? Do you you expect you could grab the constructor + third and fourth args as an as-pattern?

Haskell's pattern matching wants to keep it simple. If you want a binding to the constructor applied to an arbitrary subset of arguments, use a lambda expression mentioning your previously-bound var(s):

mkPy = \ x5 -> MkPoint x5 y1          -- y1 bound as per your q
user_28405420
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