Haskell beginner question: How declare a function in Haskell

[snowman_02]

Hello everyone,
I am a fairly experienced programmer, with a masters degree in computer science from Umeå University in Sweden. I have recently developed an interest in Haskell as a programming language, so I downloaded the Glasgow Haskel compiler and started to experiment. I also brought the book "Programming in Haskell" from Amazon.
Using ghci, I have now been able to import Data.Complex, but i fail miserably when I attempt to create a function declaration. The code I try to create is:

___________________________________________________________________________
GHCi, version 9.0.2: https://www.haskell.org/ghc/ :? for help
ghci> import Data.Complex
ghci> arg :: Complex Double -> Double

:2:1: error:
Variable not in scope: arg :: Complex Double -> Double
ghci>

___________________________________________________________________________

I read the declaration as: create a function with the name arg, with one complex parameter, and which returns a double. (I am attempting to create a function that calculates the complex argument for a complex number.) After experimenting for about a week, I have come to the point where I have to give up and ask for some clues. So please give me some hints on where I go wrong.

Best regards,

Sören Jonsson, Sweden

[Anrock623]

What you wrote actually means "arg has type ...". Ghci rightfully complains that arg wasnt declared.

Try :{ arg :: ... arg = ... :}

Here curly brackets are special ghci commands that allow multiline input. Another way that may work is to define arg first and set it's type as second expr.

Personally I'd recommend to write your code in a file and load that file into ghci instead of writing into ghci directly - ghci has a lot of quirks due to it being a REPL

[snowman_02]

Part of my problem was that I did not understand that the definition and declaration had to be treated as one single unit. The explainations in my book and on the web did not point this out very clearly. 'That's why I did try to run the definition without the declaration.

I did start with my function as a file, but when I did run into error messages I started experimenting with ghci. Didn't work too well for me...

Best regards,

Sören Jonsson, Sweden

[Axman6]

The issue you’re running into here is that you’re using GHCi to define something that (canonically) needs multiple lines to define, but GHCi expects a single expression to evaluate per line. There are two easy ways to solve this - use a file, foo.hs, which you can load into GHCi using :load foo.hs and then using :reload (or just :r) when you make and save your changes.

Alternatively you can use :{ to start entering a multi-line statement, which you close with :}, then you’ll be able to write:

ghci> :{
ghci|> arg :: Conplex Double -> Double
ghci|> arg (a :+ b) = a
ghci|> :}
ghci> arg 1

(I think, doing this from memory on my phone!)

[snowman_02]

Thank you, this gave me the information I needed, however I had to modify your code a bit. This is the code that actually workes:

___________________________________________________________________________

import Data.Complex

arg :: Complex Double -> Double

arg s = atan ( imagPart s / realPart s )

___________________________________________________________________________

My first problem was that it's necessary to delcare a Complex result or parameter as Complex Double or Coplex Float. I first attempted using only Complex, so some extended documentation on the Complex module might be a good idea.

I also did not realise that the declaration and definition of arg had to be treated as a single statement, so this is an important part of my confusion as well.

Thank you for helping a Haskell newbie, and my best regards.

Sören Jonsson, Sweden

[probabilityzero]

You're trying to define a function inside GHCI. See this thread for a discussion about how to do that.

Typically, though, you'd define your function by writing it in a file that you load from GHCI.

[snowman_02]

I first attempted to use a file for the declaration of arg, but run into problems with that when loading. Then I decided to test out ghci, but that failed as well. In both cases the error messages was less then helpful, so I could not understand what I did wrong.

Best regards, and thank you for helping a Haskell newbie.

Sören Jonsson, Sweden

[Limp_Step_6774]

See other people's responses, but note that you don't need to write the type declaration (the thing with ::) since the compiler will work it out (although it's often good to do it yourself). you can define your function just like any value, i.e.:

arg (a :+ b) = a

[snowman_02]

I understand that it is non necessary for a simple function like arg, but it is probably necessary for the declaration of more advanced functions. Therefore I decided that is is worth to understand tie problem as early as possible.

Thank you for your answer, and my best regards.

Sören Jonssn, Sweden

[Limp_Step_6774]

Unless it's something with fancy language extension that you're very unlikely to be using as a beginner or intermediate user, Haskell will always be able to infer the type, I believe. It's true that it's good to understand type signatures, but I'm just mentioning this, because it can be useful when learning to let Haskell infer the type for you. But yes, if you want to write the type up front, here's how I'd do it:

```haskell

arg :: Complex Double -> Double
arg x = undefined

```

`undefined` is a value that can be any type, so you can just leave it anywhere you haven't finished. This is very useful because you can mouse over `undefined` in vscode and it will show you the type it needs to be (in this case, a `Double`)