I am trying to implement a function that checks that all items of a list are equal to a given value. The function should take a list of strings, returning True if every string is exactly "car", and False otherwise.

*Main> go ["car","car","car"]
True

*Main> go ["car","car","bus"]
False

This is my current implementation but it does not seem to work correctly.

go :: [String] -> Bool
go ("car":[]) = True
go ("car":xs) = False
go _ = False

CodePudding user response：

The line go ("car":xs) = False means that a list that contains as first item "car" and has at least one extra item (since the pattern before covers the case of a singleton list) should return False. You should recurse on the list, so the function should look like:

go :: [String] -> Bool
go ("car" : xs) = …
go (_ : _) = False
go [] = True

where you still need to fill in the … part, this should make a recursive function call.

You can however implement go with all:: Foldable t => (a -> Bool) -> t a -> Bool:

go :: Foldable f => f String -> Bool
go = all ("car" ==)

CodePudding user response：

These are what you get from your code:

• with go ("car":[]) = True, you get

  go ["car"] -> True

• with go ("car":xs) = False

("car":xs) is a pattern matching that captures a list of the form ["car",...], so you will get

go ["car"]        -> False   -- not used because the first case takes priority
go ["car", "car"] -> False   -- INCORRECT
go ["car", "bus"] -> False
go ["car", ...]   -> False   -- NOT ALWAYS CORRECT

• with go _ = False

_ is a pattern matching that captures any list, so you will get

 go []             -> False
 go ["car"]        -> False  -- not used because the first case takes priority
 go ["bus"]        -> False
 go ["car", ...]   -> False  -- not used because the second case takes priority
 go [...]          -> False

You can get a correct answer with

 go [] = True                       
 go ("car":xs)  = ...
 go _ = False

but you need to fill ... with something that depends on xs (the tail of the list). The head (first element) of ("car":xs) is already equal to "car".