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Guide to YSH Error Handling

There are just a few concepts to know:

• error builtin - "Throw" an error, with a custom message, error code, and other properties.
• try builtin - Run a block, and set the _error register to a Dict.
  • _error.code will be 0 on success, or non-zero if an error is thrown in the block.
• failed builtin - A handy shortcut to test for a non-zero error code.

Examples

Handle command and expression errors with try

Here's the most basic form:

try {
  ls /zz
}
if (_error.code !== 0) {
  echo "ls failed with $[_error.code]"
} 
# => ls failed with error 2

The failed builtin is a shortcut

Instead of writing if (_error.code !== 0), you can write if failed:

if failed {
  echo "ls failed with $[_error.code]"
} 

This saves you 7 punctuation characters: ( _ . !== )

Use a case statement if it's not just pass/fail

Sometimes it's nicer to use case rather than if:

try {
  grep '[0-9]+' foo.txt
}
case (_error.code) {
  (0)    { echo 'found' }
  (1)    { echo 'not found' }
  (else) { echo 'error invoking grep' }
}

Error may have more attributes, like _error.message

try {
  var x = fromJson('{')
}
if failed {
  echo "JSON failure: $[_error.message]"
}
# => JSON failure: expected string, got EOF

The error builtin throws custom errors

A non-zero exit code results in a simple shell-style error:

proc simple-failure {
  return 2
}

try {
  simple-failure
}
echo "status is $[_error.code]"
# => status is 2

The error builtin is more informative:

proc better-failure {
  error 'Custom message' (code=99, foo='zz')
}

try {
  better-failure
}
echo "$[_error.code] $[_error.message] foo=$[_error.foo]"
# => 99 Custom message foo=zz"

Tips

Proc Return Status Should Be Either OK-Fail, or True-False-Fail

That is, use one of these styles:

For example, here's a proc that does is not follow the style:

proc is-different (left, right) {
  mkdir /tmp/dest            # may return 1 on failure

  cp $left $right /tmp/dest  # may return 1 on failure

  diff -u $left $right       # 0-true, 1-false, 2-failure
}

The exit code isn't well-defined, because mkdir and cp use the OK-fail paradigm, while diff uses the boolean paradigm:

Explicitly checking for failure fixes it:

proc different (left, right) {
  if ! mkdir /tmp/dest {
    return 2                 # 2-failure
  }
  if ! cp $left $right /tmp/dest {
    return 2                 # 2-failure
  }

  diff -u $left $right       # 0-true, 1-false, 2-failure
}

Related

• YSH vs. Shell Idioms > Error Handling
• YSH Fixes Shell's Error Handling (errexit) - A detailed design doc