enuminmodule.ps1

##foo.psm1
#####################

$ServiceAppMap = @{
    RAS = 'Web' ## DeployWebApplication
    EDW = 'Custom' ## Has it's own Install-<App> function
    LSA = 'Custom'
    LDI = 'Custom'
    LDA = 'Web'
    LAE = 'Web'
}

## Make the enum for param validation
Invoke-Expression ('enum GHIApplication {{{0}}}' -f [string]$ServiceAppMap.Keys.foreach({ "$_;" }))

function thing {
   param(
      [GHIApplication]$app
  )
}
######################

PS> Import-Module foo.psm1
PS> thing
Unable to find type [GHIApplication]

## If I manually run `Invoke-Expression ('enum GHIApplication {{{0}}}' -f [string]$ServiceAppMap.Keys.foreach({ "$_;" }))` 
## in the console and then try to run `thing`, it then finds the type. 
## It’s acting like during module import, it doesn’t create the enum at all.