# Issues with data types

**URL:** <https://discourse.haskell.org/t/issues-with-data-types/876>\
**Category:** Learn\
**Created:** [September 10, 2019, 10:13pm UTC](https://discourse.haskell.org/t/issues-with-data-types/876 "2019-09-10T22:13:43Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![Franpg995](https://avatars.discourse-cdn.com/v4/letter/f/9de0a6/32.png) [@Franpg995](https://discourse.haskell.org/u/Franpg995)\
**Post date:** [September 10, 2019, 10:13pm UTC](https://discourse.haskell.org/t/issues-with-data-types/876/1 "2019-09-10T22:13:43Z")

</div>

Hello, i’m having a trouble where I need to create a function that uses data types that I created and I can’t understand the error that ghci throws  
Here is my code:  
data Carrera = Matematica | Fisica | Computacion | Astronomia | Profesorado  
type Ingreso = Int  
data Funcion = Teorico | Practico  
data Rol = Decanx  
| Docente Funcion  
| Estudiante Carrera Ingreso  
cuantos\_doc :: [Rol] -\> Funcion -\> Int  
cuantos\_doc [] c = 0  
cuantos\_doc (x:xs) c =case f of  
Docente c -\> 1 + cuantos\_doc xs c  
otherwise -\> cuantos\_doc xs c  
where f = x c

And here is the error

Proyecto2.hs:25:33: error:  
• Couldn’t match expected type ‘Funcion -\> t’  
with actual type ‘Rol’  
• The function ‘x’ is applied to one argument,  
but its type ‘Rol’ has none  
In the expression: x c  
In an equation for ‘f’: f = x c  
• Relevant bindings include f :: t (bound at Proyecto2.hs:25:29)  
|  
25 | where f = x c  
| ^^^  
Failed, no modules loaded.

I also tried using “case x c of…” and got this error

Proyecto2.hs:22:29: error:  
• Couldn’t match expected type ‘Funcion -\> Rol’  
with actual type ‘Rol’  
• The function ‘x’ is applied to one argument,  
but its type ‘Rol’ has none  
In the expression: x c  
In the expression:  
case x c of  
Docente c -\> 1 + cuantos\_doc xs c  
otherwise -\> cuantos\_doc xs c  
|  
22 | cuantos\_doc (x:xs) c = case x c of  
| ^^^  
Failed, no modules loaded.

Thanks in advance

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<div class="post-metadata">

**Author:** ![f-a](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/f-a/32/2740_2.png) [@f-a](https://discourse.haskell.org/u/f-a)\
**Post date:** [September 10, 2019, 10:36pm UTC](https://discourse.haskell.org/t/issues-with-data-types/876/2 "2019-09-10T22:36:46Z")

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Buenas Francisco,

The error is not related to the snipped you pasted (which loads  
correctly in ghci), but to:

```
• Relevant bindings include f :: t (bound at Proyecto2.hs:25:29)
     >
  25 | where f = x c
     > ^^^

```

No idea what x does, but as `ghci` suggests:

```
• The function ‘x’ is applied to one argument,
  but its type ‘Rol’ has none

```

`x` is not a function, so either change `x` or `f`.  
Paste the relevant bit if you are still stuck!

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<div class="post-metadata">

**Author:** ![Franpg995](https://avatars.discourse-cdn.com/v4/letter/f/9de0a6/32.png) [@Franpg995](https://discourse.haskell.org/u/Franpg995)\
**Post date:** [September 10, 2019, 10:49pm UTC](https://discourse.haskell.org/t/issues-with-data-types/876/3 "2019-09-10T22:49:29Z")

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Hey, so x its an element of a list of Rol type elements. An execution example of cuantos\_doc would be:

> cuantos\_doc [Decanx, Docente Teorico, Docente Practico] Teorico  
> 1

I double checked that the error messages correspond to the code i pasted

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<div class="post-metadata">

**Author:** ![f-a](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/f-a/32/2740_2.png) [@f-a](https://discourse.haskell.org/u/f-a)\
**Post date:** [September 10, 2019, 11:03pm UTC](https://discourse.haskell.org/t/issues-with-data-types/876/4 "2019-09-10T23:03:11Z")

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> [@Franpg995](#):
>
> I double checked that the error messages correspond to the code i pasted

I should have waited for the edit!

> [@](#):
>
> x its an element of a list of Rol type elements

This is correct, so why are you applying it to `c` in `f = x c`? `x` can’t be applied to anything, has zero arguments!  
What are you trying to do with `f`? Can you write its signature?

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<div class="post-metadata">

**Author:** ![Franpg995](https://avatars.discourse-cdn.com/v4/letter/f/9de0a6/32.png) [@Franpg995](https://discourse.haskell.org/u/Franpg995)\
**Post date:** [September 10, 2019, 11:19pm UTC](https://discourse.haskell.org/t/issues-with-data-types/876/5 "2019-09-10T23:19:55Z")

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> [@f-a](#):
>
> What are you trying to do with `f` ?

In a dumb desperate attempt to solve the problem I thought that maybe giving a variable (in this case f) instead of “x c” would do it.  
What i’m attempting to do is something like

```haskell
... x c == Docente c = 1 + ...
```

but i can’t use the Eq class and using filter is for the next exercise.

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<div class="post-metadata">

**Author:** ![f-a](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/f-a/32/2740_2.png) [@f-a](https://discourse.haskell.org/u/f-a)\
**Post date:** [September 10, 2019, 11:49pm UTC](https://discourse.haskell.org/t/issues-with-data-types/876/6 "2019-09-10T23:49:37Z")

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`x c` is a function application, applying `x` to `c` won’t work in any case.

Be careful: the `c` in `Docente c` is **not** the same `c` in `cuantos_doc (x:xs) c`.

A working version could be:

```
data Carrera = Matematica | Fisica | Computacion | Astronomia | 
Profesorado  
type Ingreso = Int
data Funcion = Teorico | Practico deriving (Eq)
data Rol = Decanx
    | Docente Funcion
    | Estudiante Carrera Ingreso

cuantos_doc :: [Rol] -> Funcion -> Int
cuantos_doc [] c = 0
cuantos_doc (x:xs) c =
        case x of
          Docente k -> if k == c
                         then 1 + cuantos_doc xs c
                         else cuantos_doc xs c
          otherwise -> cuantos_doc xs c

```

Notice the `deriving Eq`.

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<div class="post-metadata">

**Author:** ![Franpg995](https://avatars.discourse-cdn.com/v4/letter/f/9de0a6/32.png) [@Franpg995](https://discourse.haskell.org/u/Franpg995)\
**Post date:** [September 10, 2019, 11:51pm UTC](https://discourse.haskell.org/t/issues-with-data-types/876/7 "2019-09-10T23:51:59Z")

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> [@f-a](#):
>
> Notice the `deriving Eq` .

That’s exactly what I can’t use, must find another way to get the same result without using Eq

---

<div class="post-metadata">

**Author:** ![f-a](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/f-a/32/2740_2.png) [@f-a](https://discourse.haskell.org/u/f-a)\
**Post date:** [September 11, 2019, 12:23am UTC](https://discourse.haskell.org/t/issues-with-data-types/876/8 "2019-09-11T00:23:14Z")

</div>

Create a helper function that pattern matches on it:

```
miProfeEsTontoYNoMeDejaUsarAEq :: Funcion -> Funcion -> Bool
miProfeEsTontoYNoMeDejaUsarAEq Teorico Teorico = True
miProfeEsTontoYNoMeDejaUsarAEq Practico Practico = True
miProfeEsTontoYNoMeDejaUsarAEq _ _ = False

```

And then `if miProfeEsTontoYNoMeDejaUsarAEq k c` …