Strict variable expansion

[zudov]

We've run into a bit of confusion (here https://github.com/KSF-Media/tuttifrutti/issues/12) with identifier parsing in our log module when using template to support variable interpolation.

While the actual issue comes from the lack of documentation in our library. I think we could eliminate such confusion by adding a "strict" mode in which, $foo won't be allowed, only ${foo}. It will also eliminate the need to escape $ with $$.

Such functionality could be added in backwards compatible way by introducing a templateStrict :: Text -> Template that has such behavior.

[trskop]

I was thinking about this for a while. My idea was to create new module Data.Text.Template.Parse that would expose following:

data ParseOptions = ParseOptions
    { bracketsRequired :: Bool
    , escapeChar :: Char
    }

data ParseError = ParseError
    { sourceLocation :: Maybe SrcLoc
    -- ^ Location of template parsing function in Haskell source code, if supported.
    , templateLocation :: (Int, Int)
    -- ^ Location in the template where parsing error was encountered.
    }

parse :: HasCallStack => ParseOptions -> Text -> Either ParseError Template
parse = parseImpl

Would something like this work for you as well?