Of what use is 'lambda'???

[Kragen Sitaker]

In article <39CF7C9F.315AAD67 at san.rr.com>, Darren New  <dnew at san.rr.com> wrote:
>Kragen Sitaker wrote:
>> "referentially transparent" is another way of saying "has no side
>> effects".  
>
>Actually, "referentially transparent" implies there are no side effects, but
>the opposite is not necessarily true. This is actually one reason I dislike
>Perl.
>
>@x = (3, 2, 1)
>$y = @x
>$z = (3, 2, 1)
>
>Here $y != $z
>
>Maybe that's just because assignment is explicitly a side-effect in Perl.
>It's hard to argue one way or the other. But the first statement still
>stands. :-)

You could argue that the commas in the two occurrences of (3, 2, 1)
were different operators because of Perl's context-sensitivity; whether
or not perl thinks so is probably arguable:

perl -MO=Terse -e '@x = (3, 2, 1); $y = @x; $z = (3, 2, 1)'
-e syntax OK
LISTOP (0xc75c0) pp_leave
    OP (0x135290) pp_enter
    COP (0xc7530) pp_nextstate
    BINOP (0xc06c0) pp_aassign [2]   (array assign)
        UNOP (0xbfc70) pp_null [141]
            OP (0xc8958) pp_pushmark
            SVOP (0xc0740) pp_const  IV (0xc7bac) 3
            SVOP (0xc0720) pp_const  IV (0xc7ba0) 2
            SVOP (0xc06e0) pp_const  IV (0xc9c50) 1
        UNOP (0xc7620) pp_null [141]
            OP (0xd20c8) pp_pushmark
            UNOP (0xc07a0) pp_rv2av [1]
                GVOP (0xc0780) pp_gv  GV (0xc9c2c) *x
    COP (0xc74a0) pp_nextstate
    BINOP (0x12fbc0) pp_sassign
        UNOP (0xc07e0) pp_rv2av [3]
            GVOP (0xc00c0) pp_gv  GV (0xc9c2c) *x
        UNOP (0xc0760) pp_null [15]
            GVOP (0xc0700) pp_gvsv  GV (0xc9c14) *y
    COP (0xc7590) pp_nextstate
    BINOP (0x144e40) pp_sassign   (scalar assign)
        LISTOP (0xc7470) pp_list  (see, this was pp_null above)
            OP (0x135308) pp_pushmark
            OP (0x12fc20) pp_null [5] (and the 3 and 2 have been optimized away)
            OP (0x144e80) pp_null [5]
            SVOP (0x144e60) pp_const  IV (0xc9c98) 1
        UNOP (0x144ea0) pp_null [15]
            GVOP (0x12fbe0) pp_gvsv  GV (0xc7bc4) *z

If they're different operators that just happen to be spelled the same
way --- like Python's list-index and create-an-anonymous-list operator,
both spelled with [] --- then you could argue that the above expression
is indeed referentially transparent, just not transparent in the sense
of "clearly readable to human beings".
-- 
<kragen at pobox.com>       Kragen Sitaker     <http://www.pobox.com/~kragen/>
Perilous to all of us are the devices of an art deeper than we ourselves
possess.
                -- Gandalf the Grey [J.R.R. Tolkien, "Lord of the Rings"]