Extract Integer from a datatype

Onni Pylvänen 0
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I know i can do something like this to get an integer out of a datatype:

[x | (PhoneNo x) <- [PhoneNo 1234]]

where PhoneNo is defined as:

data PhoneNo = PhoneNo Integer deriving (Eq, Ord)

However this is better for lists. How can I extract the integer of a single PhoneNo instance?

haskell syntax
1 Answers

Pattern matching is the canonical mechanism to deconstruct algebraic data types. You are already using it in your example: PhoneNo x is a pattern against which the value PhoneNo 1234 is matched, binding x to 1234 in the head of the list comprehension (here simply x).

Pattern matching happens a number of places in Haskell, most notably in function definitions:

extract :: PhoneNo -> Integer
extract (PhoneNo x) = x

Alternatively, you can match values against patterns in let, where and case expressions:

let (PhoneNo x) = PhoneNo 1234 in ... x ...

myFunction = ... x ...
   where
    (PhoneNo x) = PhoneNo 1234

case PhoneNo 1234 of
  PhoneNo x -> .... x ...
justkris0
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