aspatterns

Aspatterns are a feature of Haskell pattern matching that allow a value matched by a pattern to be simultaneously bound to a name and decomposed. The syntax uses the at symbol, as in name@pattern. When the pattern matches, the name refers to the entire matched value, while the components bound inside the pattern provide access to parts of the value. This is useful when a function needs to inspect parts of a value while also keeping a reference to the original value for further processing.

Common usage examples illustrate the idea. For instance, lengthAndHead xs@(y:_) = (length xs, y) binds xs to the whole list and y to its first element, giving both the full structure and the head in one binding. describeList s@(x:xs) = "The list starts with " ++ show x binds the entire list to s while x and xs expose the first element and the rest. A simple nonempty check can also use an as-pattern: hasHead xs@(_:_) = True, hasHead [] = False binds the whole nonempty list to xs while matching its head and tail in the pattern.

Aspatterns are applicable in function equations and case expressions, enabling concise code that both analyzes and preserves the original value. They are particularly handy when error messages, logging, or further processing require the original input alongside its decomposed components.

Limitations and considerations include potential readability concerns if overused; alternatives such as separate bindings with let or case expressions can be clearer in complex code. Aspatterns are a syntactic convenience in pattern matching and do not introduce new runtime behavior beyond binding names to values. They are a standard tool in Haskell for working with structured data while retaining the whole value for reuse.