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What is this form of pattern matching called:Option{..} <- ..., e.g. as it is used here:
data Option = Option { cabal :: Maybe String , noStylish :: Bool }
...
main = do
Option{..} <- cmdArgs defOption
cabp <- case cabal of
Nothing -> do
...
It seems to redefine cabal and nostylish. Before the pattern match cabal has type Option -> Maybe String but after it has type Maybe String.
This example comes from the recently uploaded package cabal2ghci.
468
For example, x* matches any number of x characters, [0-9]* matches any number of digits, and . * matches any number of anything. A regular expression pattern match succeeds if the pattern matches anywhere in the value being tested.
In computer science, pattern matching is the act of checking a given sequence of tokens for the presence of the constituents of some pattern.
Pattern matching is used by the shell commands such as the ls command, whereas regular expressions are used to search for strings of text in a file by using commands, such as the grep command.
This is a GHC syntactic extension called record wildcards. Quoting documentation:
Record wildcard syntax permits a
".."in a record pattern, where each elided fieldfis replaced by the patternf = f.
So this code is equivalent to
Option { cabal = cabal, noStylish = noStylish } <- cmdArgs defOption
effectively binding name x to the value of record field named x for every field in the record type.
<- part is irrelevant here, you can as well write
let Option { .. } = some expression
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