Haskell - Enforcing Naming Convention with Parsec
Ken Aguilar
Posted on September 5, 2020
In my project migratum I wanted to enforce a naming convention like this
V<version number>__<file name>.sql
+ + + + + +
| | | | | |
| | | | | v
| | | | | Enforce sql extension.
| | | | v
| | | | Enforce this dot to indicate the end of the file name.
| | | v
| | | File name need to be alpha numeric. No other symbols, except underscore
| | v
| | Enforce the double underscore
| v
| Version number have to be numbers.
v
Enforce the V character to signify "version"
Ways I can accomplish these are with
- Regex
- Parser Combinators
I'm not sure how to do this with regex because to be honest, I know very simple regex and I don't have a regex license. I'm decent with parser combinators and plus it's the title of this blog post, so that's what I'm going to use to accomplish the task I set out for.
Parser combinators really shine when you parse something recursive like json. You know how json can contain an array and objects and then these objects and arrays can contain arrays and objects, so on and so forth? Yeah, parser combinators are really neat for that. So don't judge parser combinators solely on this blog post. I chose parser combinators because it's my go-to method for parsing, and I personally think they're convenient.
Before we touch parsing, let's create a type representation of the file name based on the ascii diagram above.
data FilenameStructure
= FilenameStructure FileVersion Underscore Underscore FileName Dot FileExtension
deriving ( Eq, Show )
newtype FileVersion = FileVersion Text
deriving ( Eq, Show )
newtype Underscore = Underscore Text
deriving ( Eq, Show )
newtype FileName = FileName Text
deriving ( Eq, Show )
newtype Dot = Dot Text
deriving ( Eq, Show )
newtype FileExtension = FileExtension Text
deriving ( Eq, Show )
Now, we can focus on specific segments of the file name instead of thinking of parsing the entire file name. I think that makes it a bit easier. At least to me it does. I can think of parsing the file version, underscore, filename, etc individually.
Let's start with the imports. Popular libraries that come to mind are parsec, megaparsec, and attoparsec. Evaluate which one suits your project, but on this blog post we're using parsec.
-- parsec
import Text.ParserCombinators.Parsec (GenParser, ParseError)
import qualified Text.ParserCombinators.Parsec as Parsec
-- text
import qualified Data.Text as T
Let's start with the file version parser. Our convention says that we need to start with the character V, then it should be followed by numbers.
fileVersionParser :: GenParser Char st FileVersion
fileVersionParser = do
vChar <- Parsec.char 'V'
vNum <- Parsec.digit
pure $ FileVersion $ T.pack $ ( vChar : vNum : [] )
If we try this in ghci it will look like this
ghci> import Text.ParserCombinators.Parsec
ghci> parse fileVersionParser "" "V69"
ghci> Right ( FileVersion "V69" )
That's what we want, to match Right and return our FileVersion with
the text "V69".
ghci> parse fileVersionParser "" "Vwhat"
ghci> Left (line 1, column2):
unexpected "w"
That's right, it should fail when there's no version number.
ghci> parse fileVersionParser "" "V1what"
ghci> Right ( FileVersion "V1" )
It drops the input that isn't a number.
ghci> parse fileVersionParser "" "what"
ghci> Left (line 1, column 1):
unexpected "w"
expecting "V"
Finally, it will fail if it doesn't find the "V" character.
Woohoo! One parser down, four to go!
Let's do the rest of the parsers, and feel free to try these out in ghci.
underscoreParser :: GenParser Char st Underscore
underscoreParser = Underscore . T.pack . pure <$> Parsec.satisfy isUnderscore
isUnderscore :: Char -> Bool
isUnderscore char = any ( char== ) ( "_" :: String )
fileNameParser :: GenParser Char st FileName
fileNameParser = FileName . T.pack
<$> Parsec.many ( Parsec.alphaNum <|> Parsec.satisfy isUnderscore )
dotParser :: GenParser Char st Dot
dotParser = Dot . T.pack . pure <$> Parser.char '.'
fileExtensionParser :: GenParser Char st FileExtension
fileExtensionParser = FileExtension. T.pack <$> Parsec.string "sql"
Then, to create a parser for the whole file name we combine the rest of our parsers.
filenameStructureParser :: GenParser Char st FilenameStructure
filenameStructureParser = FilenameStructure
<$> fileVersionParser
<*> underscoreParser
<*> underscoreParser
<*> fileNameParser
<*> dotParser
<*> fileExtensionParser
We can also do this with the do syntax if that's more convenient.
filenameStructureParser :: GenParser Char st FilenameStructure
filenameStructureParser = do
version <- fileVersionParser
u1 <- underscoreParser
u2 <- underscoreParser
fileName <- fileNameParser
dot <- dotParser
ext <- fileExtensionParser
pure $ FilenameStructure version u1 u2 fileName dot ext
Finally, our parser "runner"
namingConvention :: String -> Either ParseError FilenameStructure
namingConvention =
Parsec.parse filenameStructureParser "Error: Not following naming convention"
Instead of manually trying them out in ghci we can instead do some unit testing. So we don't have to keep messing with the terminal.
module NamingConventionSpec where
import Test.Hspec
spec :: Spec
spec = do
desribe "Filename" $ do
it "follows naming convention" $ do
successResult <- pure $ namingConvention "V1__uuid_extension.sql"
emptyFileName <- pure $ namingConvention ""
noExtension <- pure $ namingConvention "V1__uuid_extension"
noVersion <- pure $ namingConvention "uuid_extension.sql"
upperCaseFileName <- pure $ namingConvention "V1_UUID_extension.sql"
symbolFileName <- pure $ namingConvention "V1_UUID+extension.sql"
-- success cases
shouldBe successResult ( Right "V1__uuid_extension.sql" )
shouldBe upperCaseFileName ( Right "V1__UUID_extension.sql" )
-- failure cases
shouldBe ( isLeft emptyFileName ) True
shouldBe ( isLeft noFileExt ) True
shouldBe ( isLeft noVersion ) True
shouldBe ( isLeft symbolFilename ) True
Now that we have our parser we can use it check for duplicates, because our types have an Eq instance we can do equality checking.
fileVersion :: FilenameStructure -> FileVersion
fileVersion ( FilenameStructure v _ _ _ _ _ ) = v
getVersion :: Either ParseError FilenameStructure -> Either ParseError FileVersion
getVersion res = case res of
Left err -> Left err
Right fs -> Right $ fileVersion fs
checkDuplicate :: [ String ] -> Either ParseError [ String ]
checkDuplicate filenames = do
versions <- traverse getVersion ( namingConvention <$> filenames )
if anySame versions
-- Express your errors however you want, this is just to show that this is
-- the error branch.
then error "Duplicate migration file"
else Right filenames
where
anySame :: Eq a => [ a ] -> Bool
anySame = isSeen []
isSeen :: Eq a => [ a ] -> [ a ] -> Bool
isSeen seen ( x:xs ) = x `elem` seen || isSeen ( x:seen ) xs
isSeen _ [] = False
When creating your parsers, avoid doing any validation or any "smart" logic. Concentrate on parsing the input and nothing more. Once you have your parsers then you can do whatever you want with the output.
References
Posted on September 5, 2020
Join Our Newsletter. No Spam, Only the good stuff.
Sign up to receive the latest update from our blog.