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Conditional

Aqua supports branching, you can:

Contract​

  • The second branch of the conditional operator is executed if and only if the first block failed.
  • The second branch has no access to the first branch's data.
  • A conditional block is considered "executed" if and only if any branch was executed successfully.
  • A conditional block is considered "failed" if and only if the second (recovery) branch failed.

Conditional operations​

if​

The first branch of the conditional operator, executed only if the condition holds.

aqua
x = true
if x:
  -- always executed
  foo()
if x == false:
  -- never executed
  bar()
if x != false:
  -- always executed
  baz()
aqua
x = true
if x:
  -- always executed
  foo()
if x == false:
  -- never executed
  bar()
if x != false:
  -- always executed
  baz()

Any expression that produces bool is acceptable as a condition.

if corresponds to match, mismatch extension of π-calculus.

else​

The second branch of if, executed only in case the condition does not hold.

aqua
if false:
  foo() -- skipped
else:
  bar() -- executed
aqua
if false:
  foo() -- skipped
else:
  bar() -- executed

If you want to set a variable based on condition, see conditional return.

try​

Tries to perform operations, swallows produced error (if there's no catch, otherwise executes catch).

aqua
try:
   -- If foo fails with an error, execution will continue
   -- You should write your logic in a non-blocking fashion:
   -- If your code below depends on `x`, it may halt as `x` is not resolved.
   -- See conditional return below for workaround
   x <- foo()
aqua
try:
   -- If foo fails with an error, execution will continue
   -- You should write your logic in a non-blocking fashion:
   -- If your code below depends on `x`, it may halt as `x` is not resolved.
   -- See conditional return below for workaround
   x <- foo()

catch​

Catches the standard error from the try block.

aqua
try:
   foo()
catch e:
   logError(e)
aqua
try:
   foo()
catch e:
   logError(e)

Type of e is:

aqua
data LastError:
  instruction: string -- What AIR instruction failed
  msg: string -- Human-readable error message
  peer_id: string -- On what peer the error happened
aqua
data LastError:
  instruction: string -- What AIR instruction failed
  msg: string -- Human-readable error message
  peer_id: string -- On what peer the error happened

otherwise​

You may add otherwise to provide recovery for any block or expression:

aqua
x <- foo()
otherwise:
  -- if foo can't be executed, then do bar()
  y <- bar()
aqua
x <- foo()
otherwise:
  -- if foo can't be executed, then do bar()
  y <- bar()

Conditional return​

In Aqua, functions may have only one return expression, which is the very last.

So to get the value based on condition, we need to use a writeable collection.

aqua
-- result may have 0 or more values of type string and is writeable
resultBox: *string
try:
  resultBox <- foo()
otherwise:
  resultBox <- bar()
-- now result contains only one value, let's extract it!
result = resultBox!
-- Type of result is string
-- Please note that if there were no writes to resultBox, 
-- the first use of result will halt.
-- So you need to be careful about it and ensure that there's always a value.
aqua
-- result may have 0 or more values of type string and is writeable
resultBox: *string
try:
  resultBox <- foo()
otherwise:
  resultBox <- bar()
-- now result contains only one value, let's extract it!
result = resultBox!
-- Type of result is string
-- Please note that if there were no writes to resultBox, 
-- the first use of result will halt.
-- So you need to be careful about it and ensure that there's always a value.