support Maybe before the type

[gregwebs]

Rather than

  Person
      name    Text Maybe

prefer

  Person
      name    (Maybe Text)

[snoyberg]

Closing out old issues, please reopen if still relevant

[gregwebs]

I would like to add this as an enhancement since it seems to be a point of a confusion for every persistent user.

[parsonsmatt]

This one is actually pretty easy.

We need an instance of PersistFieldSql (Maybe a) and then we need to change the mkColumns function to check if Maybe is the first type constructor.

[danbroooks]

@parsonsmatt can I assign myself to this? I can pick this up once the dust has settled with your current changes to the TH/Quasi modules. I was pondering how you would go about doing this today funnily enough!

[parsonsmatt]

yeah definitely!

In Database.Persist.Sql.Internal,. we have:

mkColumns = -- snip ...
    go :: FieldDef -> Column
    go fd =
        Column
            { cName = fieldDB fd
            , cNull = nullable (fieldAttrs fd) /= NotNullable || isEntitySum t
            , cSqlType = fieldSqlType fd
            , cDefault = defaultAttribute $ fieldAttrs fd
            , cGenerated = fieldGenerated fd
            , cDefaultConstraintName =  Nothing
            , cMaxLen = maxLen $ fieldAttrs fd
            , cReference = mkColumnReference fd
            }

We need to ensure that cNull is set for a column

Right now, it just sees if Maybe is present in the FieldAttrs. But we could also look at the fieldType -

isMaybe :: FieldType -> Bool
isMaybe ft =
    case ft of
        FTApp (FTTypeCon _ "Maybe") _ ->
            True
        _ ->
            False