埃菲尔铁塔：古典式压铸开关结构，附带并检查

Question

What is the best practice to do something such as

local
    l_pet: ANIMAL
do
    l_pet := catch_it_from_the_sky
    inspect l_pet
    when attached {DOG} l_pet as l_dog
        l_dog.eat (meat)
    when attached {FISH} l_pet as l_fish
        l_fish.eat (plants)
    else
        io.put_string ("Strange animal how do I feed him???")
    end
do

the compiler is complaining with the attached after when...

Update: why such a need?

because it just happened me to mess up with repeated copy-paste which is what a language tries to help avoiding. In the above case, the l_pet is written one time, with a N times if/else I'd have to write it as much times as ifs...

Answer 1

An inspect statement allows for checking if an expression has a specific value, and can be applied to expressions of integral types (such as INTEGER_64 , CHARACTER_32 or NATURAL_8 ):

inspect age
when 6 .. 16 then ...
when 18 then ...
when 80, 90 then ...
...
end

For discriminating over object types, conditional instructions are used:

if attached {DOG} pet as dog then
   dog.eat (meat)
elseif attached {FISH} pet as fish then
   fish.eat (plants)
else
   io.put_string ("Strange animal how do I feed him???")
end

Answer 2

In a multi-branch instruction

inspect exp when ... then ... else ... end

The exp expression needs to be a character or an integer expression.

In your given example I don't see the need to do that Object-Test , but if you need to do something like that you need to use the conditional instruction.

if ... then ... elseif ... then ... else ... end