# How to use the operator \<\> of overlay

**URL:** <https://uzu.lurk.org/t/how-to-use-the-operator-of-overlay/3559>\
**Category:** TidalCycles\
**Tags:** questions\
**Created:** [12 October 2021 12:15 UTC](https://uzu.lurk.org/t/how-to-use-the-operator-of-overlay/3559 "2021-10-12T12:15:37Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![ecmpeacebunny](https://uzu.lurk.org/letter_avatar_proxy/v4/letter/e/f1d935/32.png) [@ecmpeacebunny](https://uzu.lurk.org/u/ecmpeacebunny)\
**Post date:** [12 October 2021 12:15 UTC](https://uzu.lurk.org/t/how-to-use-the-operator-of-overlay/3559/1 "2021-10-12T12:15:38Z")

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Hello, I'd like to ask you a question about overlay.

`d1 $ sound ( overlay "bd sn:2" "cp*3" )`  
I hear it's the same as `d1 $ sound ("bd sn:2" <> "cp*3")`  
I'm going to use `<>` in the tidal cycle, but it is not recognized.  
How can overlay operator `<>` use code?

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**Author:** ![fnab](https://uzu.lurk.org/user_avatar/uzu.lurk.org/fnab/32/2083_2.png) [@fnab](https://uzu.lurk.org/u/fnab)\
**Post date:** [31 October 2021 19:28 UTC](https://uzu.lurk.org/t/how-to-use-the-operator-of-overlay/3559/2 "2021-10-31T19:28:53Z")

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Hi!

what error message are you receiving? If I do this with vanilla settings I get

```haskell
    Ambiguous occurrence ‘<>’
    It could refer to
       either ‘Sound.Tidal.Context.<>’,
              imported from ‘Sound.Tidal.Context’
              (and originally defined in ‘Sound.Tidal.Core’)
           or ‘Prelude.<>’,
              imported from ‘Prelude’ (and originally defined in ‘GHC.Base’)

```

In this case, the problem is that the operator `<>` is defined both in `Prelude` (which is a set of standard functions that are automatically imported) and in `Sound.Tidal.Context` (which is the one you want to use). Consequently the compiler does not know which version of `<>` you want to use and you have to do this explicitly. To do that you have several options:

1. Write `Sound.Tidal.Context.<>` explicitly, i.e.,

```haskell
d1 $ sound ("bd sn:2" Sound.Tidal.Context.<> "cp*3")

```

which is a quick fix but is cumbersome to write every time  
2. Import prelude without `<>` to avoid the name clash. You can do this by writing

```haskell
import Prelude hiding ((<>))

```

and evaluating the line in tidal. The ambiguity should now be resolved.  
3. Define `<>` explicitly as `Sound.Tidal.Context.<>`, i.e.,

```haskell
(<>) = (Sound.Tidal.Context.<>)

```

Again, evaluate the above line in tidal and `<>` should now work as expected.

You can add (2.) or (3.) to you boot tidal to avoid having to execute them every time you launch tidal cycles.

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<div class="post-metadata">

**Author:** ![fnab](https://uzu.lurk.org/user_avatar/uzu.lurk.org/fnab/32/2083_2.png) [@fnab](https://uzu.lurk.org/u/fnab)\
**Post date:** [31 October 2021 19:37 UTC](https://uzu.lurk.org/t/how-to-use-the-operator-of-overlay/3559/3 "2021-10-31T19:37:48Z")

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Incidentally, I have a follow up question:

Why does `Sound.Tidal.Core` redefine `<>` and not simply make `Pattern a` an instance of `Semigroup`? I guess you could argue that `Pattern a` does have many binary operators that qualify as semigroup but then, why choose `<>` to be the operator for `overlay`?

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**Author:** ![yaxu](https://uzu.lurk.org/user_avatar/uzu.lurk.org/yaxu/32/2_2.png) [@yaxu](https://uzu.lurk.org/u/yaxu)\
**Post date:** [1 November 2021 20:19 UTC](https://uzu.lurk.org/t/how-to-use-the-operator-of-overlay/3559/4 "2021-11-01T20:19:37Z")

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Hi @fnab,  
There is no good reason, apart from I don't really know what `Semigroup` is. I remember there was some structural change with this package that caused some build errors a while ago. I guess that was the point that `<>` from `Semigroup` came into scope and broke things. If `<>` has the same meaning in both packages that's a happy coincidence. I'd happily accept a pull request that defines a pattern as a semigroup instance, if that improves things!

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<div class="post-metadata">

**Author:** ![fnab](https://uzu.lurk.org/user_avatar/uzu.lurk.org/fnab/32/2083_2.png) [@fnab](https://uzu.lurk.org/u/fnab)\
**Post date:** [1 November 2021 23:34 UTC](https://uzu.lurk.org/t/how-to-use-the-operator-of-overlay/3559/5 "2021-11-01T23:34:06Z")

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> I'd happily accept a pull request that defines a pattern as a semigroup instance, if that improves things!

Great! I was just looking for an easy first issue to start contributing. I'll try to do it sometime this week.

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**Author:** ![yaxu](https://uzu.lurk.org/user_avatar/uzu.lurk.org/yaxu/32/2_2.png) [@yaxu](https://uzu.lurk.org/u/yaxu)\
**Post date:** [1 November 2021 23:53 UTC](https://uzu.lurk.org/t/how-to-use-the-operator-of-overlay/3559/6 "2021-11-01T23:53:11Z")

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Excellent, looking at `Semigroup` it does look like a good first 'issue'.