I tried to create a few modification on the NFT minting contract from week 5 of the plutus pioneer program. I wanted to be able to mint a few tokens at once. My policy should require that the token names start with "ABC" + Integer where the integer is less than 100.

Here is my attempt:

{-# INLINABLE mkPolicy #-}
mkPolicy :: TxOutRef -> () -> ScriptContext -> Bool
mkPolicy oref () ctx = traceIfFalse "UTxO not consumed"   hasUTxO           &&
                          traceIfFalse "wrong amount minted" checkMintedAmount
  where
    info :: TxInfo
    info = scriptContextTxInfo ctx

    hasUTxO :: Bool
    hasUTxO = any (\i -> txInInfoOutRef i == oref) $ txInfoInputs info

    flattenVal :: [(CurrencySymbol, TokenName, Integer)]
    flattenVal = flattenValue (txInfoMint info)

    -- Check that TokenName is of the form ABC1 .. ABC99
    checkTokenName :: TokenName ->  Bool
    checkTokenName tn = isPrefixOf "ABC" (show tn) &&
                        isJust (readMaybe (post tn) :: Maybe Integer) &&
                        (read (post tn) :: Integer) < 100
        where
            post tn' = drop 3 $ show tn'

    checkMintedAmount :: Bool
    checkMintedAmount = all (\(_, tn', amt') -> checkTokenName tn' && amt' == 1) flattenVal

Unfortunately this gives a horrible run time error which would be too long to paste here, so I'll just paste the first few lines:

GHC Core to PLC plugin: E042:Error: Unsupported feature: Int: use Integer instead
Context: Compiling type: GHC.Types.Int
Context: Compiling type: GHC.Types.Int -> a -> GHC.Show.ShowS
Context: Compiling data constructor type: GHC.Show.C:Show
Context: Compiling type: GHC.Show.Show a
Context: Compiling expr: \ ($dShow_a14Ch [Occ=Once*] :: GHC.Show.Show a)
                           (tn' [Occ=Once*] :: a) ->
                           let {
                             n :: GHC.Types.Int
                             [LclId,
                              Unf=Unf{Src=<vanilla>, TopLvl=False, Value=True, ConLike=True,
                                      WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 20}]
                             n = GHC.Types.I# 3# } in
                           letrec {
                             unsafeDrop [Occ=LoopBreaker]
                               :: forall a1. GHC.Types.Int -> [a1] -> [a1]
                             [LclId,
                              Arity=2,
                              Unf=Unf{Src=<vanilla>, TopLvl=False, Value=True, ConLike=True,
                                      WorkFree=True, Expandable=True, Guidance=IF_ARGS [20 30] 91 10}]
                             unsafeDrop
                               = \ (@ a1)
                                   (ds [Occ=Once!] :: GHC.Types.Int)
                                   (ds1 [Occ=Once!] :: [a1]) ->
                                   case ds of { GHC.Types.I# ipv ->
                                   case ds1 of {
                                     [] -> GHC.Types.[] @ a1;
                                     : _ [Occ=Dead] ipv2 [Occ=Once*] ->
                                       case ipv of {
                                         __DEFAULT ->
                                           unsafeDrop @ a1 (GHC.Types.I# (GHC.Prim.-# ipv 1#)) ipv2;
                                         1# -> ipv2
                                       }
                                   }
                                   }; } in
                           case GHC.Classes.leInt n (GHC.Types.I# 0#) of {
                             GHC.Types.False ->
                               unsafeDrop @ GHC.Types.Char n (GHC.Show.show @ a $dShow_a14Ch tn');
                             GHC.Types.True -> GHC.Show.show @ a $dShow_a14Ch tn'
                           }

I saw similar discussion on discord about trying to compare the token name to an empty string. Is this somehow related? I though the above would work because I'm explicitly doing show on the BuiltinByteString

Any idea what I'm doing wrong?