# Type variable dom0 is ambiguous when using DomainPeriod

**URL:** <https://clash-lang.discourse.group/t/type-variable-dom0-is-ambiguous-when-using-domainperiod/28>\
**Category:** Clash Help\
**Created:** [June 11, 2024, 12:07pm UTC](https://clash-lang.discourse.group/t/type-variable-dom0-is-ambiguous-when-using-domainperiod/28 "2024-06-11T12:07:31Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![jvnknvlgl](https://yyz2.discourse-cdn.com/free1/user_avatar/clash-lang.discourse.group/jvnknvlgl/32/14_2.png) [@jvnknvlgl](https://clash-lang.discourse.group/u/jvnknvlgl)\
**Post date:** [June 11, 2024, 12:07pm UTC](https://clash-lang.discourse.group/t/type-variable-dom0-is-ambiguous-when-using-domainperiod/28/1 "2024-06-11T12:07:31Z")

</div>

I’m using DomainPeriod to create a type based on Index that holds a maximum value for a 1.6 ms timer like this:

```haskell
type Timeout dom = Index (DivRU (1600000000) (Max 1 (DomainPeriod dom)))

```

However, when I now try to use this type I get the following error:

```auto
    • Couldn't match type: Div
                             (1600000000
                              + (Max
                                   1
                                   (Clash.Signal.Internal.DomainConfigurationPeriod
                                      (KnownConf dom0))
                                 - 1))
                             (Max
                                1
                                (Clash.Signal.Internal.DomainConfigurationPeriod (KnownConf dom0)))
                     with: Div
                             (1600000000
                              + (Max
                                   1
                                   (Clash.Signal.Internal.DomainConfigurationPeriod (KnownConf dom))
                                 - 1))
                             (Max
                                1
                                (Clash.Signal.Internal.DomainConfigurationPeriod (KnownConf dom)))
      Expected: Timeout dom
        Actual: Timeout dom0
      NB: ‘Div’ is a non-injective type family
      The type variable ‘dom0’ is ambiguous
    • In the ambiguity check for ‘timeout’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      In the type signature: timeout :: (KnownDomain dom) => Timeout dom

```

This error occurs in the following code snippet:

```haskell
-- | The duration of @linkTimer@ is 1.6 ms according to the SGMII reference,
-- which means that it has a frequency of 625 Hz. This is the same as 200000
-- cycles of the 125 MHz clock: @1.6*10^−3 / (1 / (125×10^6))@.
--
-- For simulation and testing, this is set to a more reasonable amount of 3
-- to decrease the amount of test values that are needed to trigger a timeout.
timeout :: (KnownDomain dom) => Timeout dom
timeout = if clashSimulation then 3 else maxBound

```

---

<div class="post-metadata">

**Author:** ![Rowan](https://yyz2.discourse-cdn.com/free1/user_avatar/clash-lang.discourse.group/rowan/32/24_2.png) [@Rowan](https://clash-lang.discourse.group/u/Rowan)\
**Post date:** [June 12, 2024, 1:54am UTC](https://clash-lang.discourse.group/t/type-variable-dom0-is-ambiguous-when-using-domainperiod/28/2 "2024-06-12T01:54:46Z")

</div>

the result type of `timeout` cannot be inferred from any input. You can enable the `AllowAmbiguousTypes` language extension to allow this, in which case you must use a type application or annotation. Or you can add a proxy argument to carry the type.

```Haskell
timeout :: (KnownDomain dom) => Proxy dom -> Timeout dom

```

---

<div class="post-metadata">

**Author:** ![martijnbastiaan](https://yyz2.discourse-cdn.com/free1/user_avatar/clash-lang.discourse.group/martijnbastiaan/32/11_2.png) [@martijnbastiaan](https://clash-lang.discourse.group/u/martijnbastiaan)\
**Post date:** [June 12, 2024, 8:46am UTC](https://clash-lang.discourse.group/t/type-variable-dom0-is-ambiguous-when-using-domainperiod/28/3 "2024-06-12T08:46:08Z")

</div>

To expand on Rowan’s answer a bit, the reason result type cannot be inferred is because the “resolved” type of `Timeout` doesn’t mention `dom` at all.

In other words, what the type checker might try to do is try and unify `Index 5` and `Timeout dom`. So it will try to answer: given that `Timeout dom ~ Index 5`, what is `dom`? And the answer is: there could be many different `dom`s that fit that bill, hence, `dom` is ambiguous.