Using Terraform For_Each and For commands to filter elements

I have a Terraform locals.tf file like the below snippet, which contains my environment definition.

locals {
    environments = [
    "dev", 
    "test", 
    "preprod",
    "prod"
   ]
}

I am now looking to create an identical resource, an Azure monitor metric alert in this case, for each of the listed environments (except prod). I am attempting to do so using a For_Each loop in the resource block of my main.tf file, a snippet of which is shown below:

resource "azurerm_monitor_metric_alert" "main" {
  for_each = {
     [for s in toset(local.environments) : if s != "prod"] 
  }   
  name = "${each.key}-metric-alert"
  resource_group_name = azurerm_resource_group.rg.name
  scopes              = [azurerm_storage_account.to_monitor[each.key].id]
  [..........ADDITIONAL NON-PROD RESOURCE CONFIG...........]

For the prod environment though, the monitor alert requires a slightly different set of configuration from the others, and so what I'd like to do is filter out that environment from the locals.tf file and then using another azurerm_monitor_metric_alert resource block, configure it along the lines of the below snippet:

   resource "azurerm_monitor_metric_alert" "main" {
      for_each = {
         [for s in toset(local.environments) : if s == "prod"] 
      } 
      name = "${each.key}-metric-alert"
      resource_group_name = azurerm_resource_group.rg.name
      scopes              = [azurerm_storage_account.to_monitor[each.key].id] 
      [..........ADDITIONAL PROD CONFIG...........]

I have the tried the above and various other implementations, but unfortunately I simply can't get this to work. Would greatly appreciate some assistance here.

So thanks to the wonderful contribution of Martin Atkins, coupled with some further research and experiments, I managed to successfully construct my solution mainly by way of the following file updates which may help others in future:

locals.tf

locals {
  environments = toset([
    "dev", 
    "test", 
    "preprod",
    "prod",
  ])
}



 

main.tf

Non-Prod Environments

resource "azurerm_monitor_action_group" "non-prod" { 
   for_each = {for p in toset(local.environments): p => p if p != "prod" }    
       name                = "${each.key}-actiongroup"
       resource_group_name = azurerm_resource_group.rg.name
       short_name          = "non-prod-acg"
       webhook_receiver {
          name        = "callmyapi"
          service_uri = "http://example.com/alert"
        }
          email_receiver {
          name          = "John Doe"
          email_address = "jdoe@myemail.com"
        }
    }
    
resource "azurerm_monitor_metric_alert" "non-prod" {
   for_each = {for p in toset(local.environments): p => p if p != "prod" }        
       name = "${each.key}-metric-alert"
       resource_group_name = azurerm_resource_group.rg.name
       scopes             = [azurerm_storage_account.to_monitor[each.key].id]
       description        = "each.value.description"
       criteria {
          metric_namespace = "Microsoft.Storage/storageAccounts"
          metric_name      = "Transactions"
          aggregation      = "Total"
          operator         = "GreaterThan"
          threshold        = 50
          dimension {
            name     = "ApiName"
            operator = "Include"
            values   = ["*"]
          }
        }
        action {
          action_group_id = azurerm_monitor_action_group.non-prod[each.key].id
        }
    }

Prod Environment

  resource "azurerm_monitor_action_group" "prod" {
    for_each = {for p in toset(local.environments): p => p if p == "prod" }   
    name                = "${each.key}-actiongroup"
    resource_group_name = azurerm_resource_group.rg.name
    short_name          = "prod-acg"
    webhook_receiver {
      name        = "callmyapi"
      service_uri = "http://example.com/alert"
    }
      email_receiver {
      name          = "D Smith"
      email_address = "dsmith44@myemail.com"
    }
  }


resource "azurerm_monitor_metric_alert" "prod" {
  for_each = {for p in toset(local.environments): p => p if p == "prod" }    
      name = "${each.key}-metric-alert"
      resource_group_name = azurerm_resource_group.rg.name
      scopes              = [azurerm_storage_account.to_monitor[each.key].id]
      description         = "${each.key} - Action will be triggered when Transactions count is greater than 60"
      criteria {
        metric_namespace = "Microsoft.Storage/storageAccounts"
        metric_name      = "Transactions"
        aggregation      = "Total"
        operator         = "GreaterThan"
        threshold        = 60
        dimension {
          name     = "ApiName"
          operator = "Include"
          values   = ["*"]
        }
      }
      action {
          action_group_id = azurerm_monitor_action_group.prod[each.key].id
      }
  }