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catch behavior unclear
# help
k
when using a catch, I get exception:
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CODE_INVOCATION_FAILED
Called block with too few arguments.
Got: 0 Expected: 1.
Target:
     testList.<block>.<block>  test.toit:32:24
I want to catch OUT_OF_BOUNDS EXCEPTION error. But I get exceptions with valid indices. What is wrong here ?
k
When you use 
catch:
, you're passing a block to the called method. That block is called without any arguments, so if the block you provide expects one or more arguments, you'll get an exception.
Your block can take arguments in two ways.
One is explicit: 
catch: | x y | ...
.
Here your block expects two arguments, so it will not work.
k
Copy code
testList:
  DATA ::= [1,2]


  2.repeat:         // works 
    d := DATA[it]
    print "$it: $d"

  2.repeat:
    d := null 
    e := catch --trace : d = DATA[it]
    print "$it: $d"
k
The other and more subtle one is the implicit one introduced when the block uses 
it
.
So here you want the outer block to take an argument, but not the inner one.
2.repeat: | index | catch --trace: DATA[index]
k
Thank you: I'm starting to understand it ...
... and it already works !
... so i had a problem with the right use of 'it' ?
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main:
  testList

testList:
  DATA ::= [1,2]

  // force out of bounds error - but catch it
  3.repeat: | index |
    d := null 
    e := catch : d = DATA[index]
    print "$index: $d"
Now it works - as expected ! Nevertheless, what is the difference in using 'it' and 'index' ?
k
Using 
it
, adds an implicitly defined parameter to the innermost block. In your case that was the block passed to 
catch:
.
After your change, you no longer use 
it
from within the block passed to 
catch:
, so it doesn't get the unwanted parameter.
When you use 
index
or any other name you explicitly give your block parameters, you do not get any implicitly defined parameters for the block passed to 
catch:
. Instead, the compiler knows that 
index
refers to the parameter in the block passed to 
repeat:
.
k
aha : from now on I will avoid 'it' and use only explicit parameters 😉
k
I mostly use 
it
for "small" blocks. That's where it really shines.
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