Matching elements of records
Fri Oct 29 12:47:27 CEST 2004
> OK. First he notes that there are three uses of "=" in Erlang.
> (1) <pattern> = <expression> "LET"
> (2) <pattern> = <pattern> "AS-pattern"
> (3) #<record>(<field> = <whatever>) "field"
> Let's dispose of (3) first. I firmly believe that it is a mistake to
> use the same symbol for (3) as for the other purposes.
I agree - when we made the record syntax we all agreed that the syntax was
terrible but we couldn't at the time think of anything better.
> (C) CONSISTENCY WITH USE (1) OF = IN ERLANG.
> To me it is simply inconsistent to bind X in the body of a function
> by writing
> X = [Y|Ys]
> and then to turn around and do the exact OPPOSITE in the head of
> a function by writing
> [Y|Ys] = X
> When he wrote
> Writing it this way means you have the same semantics as for a normal
> explicit match, take the value of "A" and match it with the record ...
> This then only gives two uses.
> Robert was, to my mind, confused. Because writing the variable to be
> bound on the right of the = is *NOT* "the same semantics as for a
> normal match" but in fact the very direct opposite.
> (This is the new reason I promised.)
> I agree 100% with Robert Virding that consistency in the uses of =
> is a good thing, but that is a reason why variable-on-the-right is
> a BAD thing and should never have been allowed.
I don't really see it like that. In Erlang you can write
Pattern1 = Pattern2 = Pattern3 = .... = X
Where X is a ground term or something which evaluates to a ground term.
I view writing
foo(Pattern1 = Pattern2 = Pattern3...)
as a kind of syntactic sugar for
Pattern1 = Pattern2 = Pattern3... = X
so if I call a function with a list
and write a head pattern like this:
I pattern match X = [H|T] = [hi,joe] which is the same as
[H|T] = X = [hi,joe]
So from that point of view I don't really care about the order of the
So in function bodies X1 = X2 = ... = Xn all the X(i)'s except Xn are patterns
and Xn is an expression, whereas in function head arguments of the form
foo(A1, A2, ...,An) -> all the Ai's are patterns.
I personally don't find this usage confusing.
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