Haskell function with no input arguments

Question

How can I define a function that takes no arguments?

data Address
   = Address
   {
       city :: String,
       street :: String
   }
   deriving ( Show )

-- mkIncompleteAddress :: () -> Address
-- mkIncompleteAddress = Address "NYC" "<TBD>"

mkIncompleteAddress :: Address
mkIncompleteAddress = Address "NYC" "<TBD>"

The closest I can get is a constant (?) global variable (?) When I remove the comments I get:

Main.hs:10:23: error:
    • Couldn't match expected type ‘() -> Address’
                  with actual type ‘Address’
    • Possible cause: ‘Address’ is applied to too many arguments
      In the expression: Address "NYC" "<TBD>"
      In an equation for ‘mkIncompleteAddress’:
          mkIncompleteAddress = Address "NYC" "<TBD>"

What do you mean by "function that takes no arguments", if not what you have here? And, once you've said what you mean, why do you want that? — Daniel Wagner
– Daniel Wagner, Commented Sep 24, 2022 at 4:00
@DanielWagner I have Addresses that are either partial or complete. At some later phase, the partial addresses will be completed. I thought I'd "allocate" a partial address and "complete" it later. I guess my thinking was "too-cpp-like" as I now see that all these "allocated address" are essentially just a single, constant address — OrenIshShalom
– OrenIshShalom, Commented Sep 24, 2022 at 7:20

bradrn · Accepted Answer · 2022-09-24 03:30:04Z

3

mkIncompleteAddress :: Address is a function which takes no arguments. In Haskell, that’s more or less the same thing as a global constant, since non-IO functions don’t have side-effects and values are lazily evaluated.

On the other hand, it’s still entirely possible to write a function mkIncompleteAddress :: () -> Address, like so:

mkIncompleteAddress :: () -> Address
mkIncompleteAddress x = Address "NYC" "<TBD>"

Or, you can be really explicit and pattern-match on the input:

mkIncompleteAddress :: () -> Address
mkIncompleteAddress () = Address "NYC" "<TBD>"

Syntactically, this looks a bit like a function with no arguments, but it isn’t: it’s a function which requires one argument, where this argument happens to have () as its only value.

However, I would invite you to reconsider your approach here. Address "NYC" "<TBD>" is, quite obviously, invalid as an address, and using such values in your program just makes mistakes easier to make. I’d recommend doing something like this instead:

mkIncompleteAddress :: String -> Address
mkIncompleteAddress street = Address "NYC" street

Or even simpler, taking advantage of currying:

mkIncompleteAddress :: String -> Address
mkIncompleteAddress = Address "NYC"

answered Sep 24, 2022 at 3:30

bradrn

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3 Comments

Jon Purdy Over a year ago

To expand on your first point: precisely speaking, Haskell functions are types that go … -> …, and they all take exactly 1 parameter; yet in practice we also often refer to a function as taking 2 or more parameters, as if it weren’t curried. So, even though it is a common misconception that a value is a 0-parameter function, I don’t think there’s really any harm in it either, as an illustration to help learn the language.

OrenIshShalom Over a year ago

Thanks ! I'll consider that idea. The only thing that bothers me is that I have some complete addresses and some partial addresses being handled by the same functions - so using your suggestion, these 2 addresses kinds will have different types, right?

bradrn Over a year ago

@OrenIshShalom If you genuinely do need to handle partial addresses so often, I’d suggest reflecting that directly in the type: something like data Address = Address { city :: String, street :: Maybe String }, perhaps.

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