W3cubDocs

Destructuring with pcase Patterns

Pcase patterns not only express a condition on the form of the objects they can match, but they can also extract sub-fields of those objects. For example we can extract 2 elements from a list that is the value of the variable my-list with the following code:

  (pcase my-list
    (`(add ,x ,y)  (message "Contains %S and %S" x y)))

This will not only extract x and y but will additionally test that my-list is a list containing exactly 3 elements and whose first element is the symbol add. If any of those tests fail, pcase will immediately return nil without calling message.

Extraction of multiple values stored in an object is known as destructuring. Using pcase patterns allows to perform destructuring binding, which is similar to a local binding (see Local Variables), but gives values to multiple elements of a variable by extracting those values from an object of compatible structure.

The macros described in this section use pcase patterns to perform destructuring binding. The condition of the object to be of compatible structure means that the object must match the pattern, because only then the object’s subfields can be extracted. For example:

  (pcase-let ((`(add ,x ,y) my-list))
    (message "Contains %S and %S" x y))

does the same as the previous example, except that it directly tries to extract x and y from my-list without first verifying if my-list is a list which has the right number of elements and has add as its first element. The precise behavior when the object does not actually match the pattern is undefined, although the body will not be silently skipped: either an error is signaled or the body is run with some of the variables potentially bound to arbitrary values like nil.

The pcase patterns that are useful for destructuring bindings are generally those described in Backquote Patterns, since they express a specification of the structure of objects that will match.

For an alternative facility for destructuring binding, see seq-let.

Macro: pcase-let bindings body…

Perform destructuring binding of variables according to bindings, and then evaluate body.

bindings is a list of bindings of the form (pattern exp), where exp is an expression to evaluate and pattern is a pcase pattern.

All exps are evaluated first, after which they are matched against their respective pattern, introducing new variable bindings that can then be used inside body. The variable bindings are produced by destructuring binding of elements of pattern to the values of the corresponding elements of the evaluated exp.

Macro: pcase-let* bindings body…

Perform destructuring binding of variables according to bindings, and then evaluate body.

bindings is a list of bindings of the form (pattern exp), where exp is an expression to evaluate and pattern is a pcase pattern. The variable bindings are produced by destructuring binding of elements of pattern to the values of the corresponding elements of the evaluated exp.

Unlike pcase-let, but similarly to let*, each exp is matched against its corresponding pattern before processing the next element of bindings, so the variable bindings introduced in each one of the bindings are available in the exps of the bindings that follow it, additionally to being available in body.

Macro: pcase-dolist (pattern list) body…

Execute body once for each element of list, on each iteration performing a destructuring binding of variables in pattern to the values of the corresponding subfields of the element of list. The bindings are performed as if by pcase-let. When pattern is a simple variable, this ends up being equivalent to dolist (see Iteration).