Conditional Operators

where clause

The where clause introduces one or more conditions that must be true for the pattern preceding it to execute.

PATTERN 
`console.log($message)` => . where $message <: js"'Hello, world!'"
INPUTOUTPUT 
console.log('Hi');
console.log('Hello, world!');
console.log('Hi');

The where clause can be followed by either a single condition or a list of conditions, wrapped in braces and separated by commas ({ cond1, cond2 }). If a where clause contains multiple conditions, all must be true for the pattern to match.

Match (<:) condition

Grit's most common condition, the match operator <: specifies what part of the codebase you want to target. It does this by matching a metavariable to its left against a pattern to its right.

GRIT 
`console.log('$message')` where $message <: `Hello, world!`

! condition

The ! operator is used to negate a condition.

PATTERN 
`console.log('$message');` => `console.warn('$message');` where {
  ! $message <: "Hello, world!"
}
INPUTOUTPUT 
console.log('Hello, world!');
console.log('Hello, people!');
console.log('Hello, world!');
console.warn('Hello, people!');

! is similar to the not modifier, except it applies to the entire condition, rather than the pattern within the condition. For example, ! $message <: "Hello, world!" is equivalent to $message <: not "Hello, world!".

and condition

The and operator is true if all of the conditions are true. Note that the and in the following example is redundant, as the where clause already requires all conditions to be true.

PATTERN 
`console.$method('$message');` => `console.warn('$message');` where {
  and {
    $message <: r"Hello, .*!",
    $method <: `log`
  }
}
INPUTOUTPUT 
console.log('Hello, world!');
console.error('Hello, world!');
console.warn('Hello, world!');
console.error('Hello, world!');

or condition

The or operator is true if any of the conditions are true.

PATTERN 
`console.$method('$message');` => `console.warn('$message');` where {
  or {
    $message <: "Hello, world!",
    $method <: `error`
  }
}
INPUTOUTPUT 
console.log('Hello, world!');
console.error('Hello, people!');
console.info('Hello, people!');
console.warn('Hello, world!');
console.warn('Hello, people!');
console.info('Hello, people!');

if as a condition

The if clause can be used to add a condition only if another condition is true.

PATTERN 
`$method('$message')` where {
  if ($message <: r"Hello, .*!") {
    $method => `console.info`
  } else {
    $method => `console.warn`
  }
}
INPUTOUTPUT 
console.log('Hello, world!');
console.log('Hello, people!');
console.log('Hi!');
console.info('Hello, world!');
console.info('Hello, people!');
console.warn('Hi!');

Rewrite (=>) as condition

Rewrites can also be used within conditions. The syntax is the same as for rewrites, but the left-hand side of the rewrite must be a metavariable.

The rewrite will be applied to the code locations referenced by the metavariable, and the condition will match if the rewrite succeeds for all locations.

PATTERN 
`console.log('$message')` where $message => `Hello, world!`
INPUTOUTPUT 
console.log('Hello?');
console.log('Hello, world!');

Assignment =

The assignment operator = is used to assign a value to a metavariable, much in the same way as you would assign to a variable in other programming languages.

The value can be anything that can appear on the right-hand side of a rewrite.

An assignment can appear inside a where clause, and always succeeds.

GRIT 
`console.log($message)` as $log where {
  $new_log_call = `logger.log($message)`,
  $log => $new_log_call
}