code: func (api *Client) SetChannelPurposeContext(ctx context.Context, channelID, purpose string) (string, error) {
  	values := url.Values{
  		"token":   {api.token},
  		"channel": {channelID},
  		"purpose": {purpose},
  	}
  
  	response, err := api.channelRequest(ctx, "channels.setPurpose", values)
  	if err != nil {
  		return "", err
  	}
  	return response.Purpose, nil
  }
docs: // SetChannelPurposeContext sets the channel purpose and returns the purpose that was successfully set with a custom context
  // see https://api.slack.com/methods/channels.setPurpose