Where

You can set variables to be used in functions and guard conditionals by using the where operator. This also called binding, because you are binding some information to the context of the function. In the example below the use of where allows use to only write the length x expression once and assign it to a variable that can then be used with each guard clause of the function.

rename  ::  String  ->  String
rename x
       | y <=  0   =  "Nemo"
       | y <  5    =  "Fiveo"
       | y <  10   =  "Decaron"
       | otherwise =  "Longbono"
       where y = length x

You can define multiple function scoped variables using where by listing other variables below the initial where clause, indented at the level of the first variable definition.

rename  ::  String  ->  String
rename x
       | y <=  nothing =  "Nemo"
       | y <  small    =  "Fiveo"
       | y <  medium   =  "Decaron"
       | otherwise     =  "Longbono"
       where y = length x
             nothing = 0
             small = 5
             medium = 10

You can use pattern matching inside a where block.

where y = length x
      (nothing, small, medium) = (0, 5, 10)

You can define functions inside a where block that can be used in the context of your main function.

initials  :: [(String, String)] -> [String]
initials names = [strip f l | (f, l) <- names]
where strip firstname lastname = [head firstname] ++  "."  ++ [head lastname]

You can nest where blocks within functions. The nested where block must be indented past the original where block’s variable definitions.

You can define pattern matched expressions as part of a where block.

describeList xs = "The list is " ++ what xs
  where what [] = "empty."
        what [x] = "a singleton list."
        what xs = "a longer list."