Ken Aguilar

Posted on Sep 5, 2020 • Updated on Sep 14, 2020

Haskell - Enforcing Naming Convention with Parsec

#haskell #parsing #functional

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

Haskell from First Principles

Monadic Parser Combinators

Top comments (2)

Simon Shine • Sep 17 '20

For comparison, this is how it might look with Megaparsec:

{-# LANGUAGE RecordWildCards #-}

module MigratumMegaparsec
  ( MigrationFile
  , prettyMigrationFile
  , parseMigrationFile
  ) where

import Control.Applicative (some)
import Data.Bifunctor (first)
import Data.Char (isAlphaNum)
import Data.Version (Version, makeVersion, showVersion)
import Data.Void (Void)

import Text.Megaparsec (Parsec, parse, chunk, takeWhile1P, (<?>))
import Text.Megaparsec.Char (char)
import Text.Megaparsec.Char.Lexer (decimal)
import Text.Megaparsec.Error (errorBundlePretty)

data MigrationFile = MigrationFile
  { mfVersion :: Version
  , mfName    :: FilePath
  } deriving (Eq, Show)

type Parser a = Parsec Void String a

prettyMigrationFile :: MigrationFile -> String
prettyMigrationFile MigrationFile{..} =
  "V" <> showVersion mfVersion <> "__" <> mfName <> ".sql"

parseMigrationFile :: String -> Either String MigrationFile
parseMigrationFile = first errorBundlePretty . parse migrationFile ""

-- V<version number>__<file name>.sql
migrationFile :: Parser MigrationFile
migrationFile =
  MigrationFile
    <$> (char 'V' *> version)
    <*> (chunk "__" *> filename <* chunk ".sql")

version :: Parser Version
version = fmap (makeVersion . pure) decimal <?> "version"

filename :: Parser FilePath
filename = takeWhile1P (Just "filename") isAlphaNum

And in action you see one benefit of using Megaparsec over Parsec or regex-applicative: Superior error messages.

λ> prettyMigrationFile <$> parseMigrationFile "V123__hello.sql"
Right "V123__hello.sql"

λ> let derp = either putStrLn (putStrLn . prettyMigrationFile) . parseMigrationFile

λ> derp ""
1:1:
  |
1 | <empty line>
  | ^
unexpected end of input
expecting 'V'

λ> derp "V"
1:2:
  |
1 | V
  |  ^
unexpected end of input
expecting version

λ> derp "V1"
1:3:
  |
1 | V1
  |   ^
unexpected end of input
expecting "__" or digit

λ> derp "V1_"
1:3:
  |
1 | V1_
  |   ^
unexpected '_'
expecting "__" or digit

λ> derp "V1__"
1:5:
  |
1 | V1__
  |     ^
unexpected end of input
expecting filename

λ> derp "V1__foo"
1:8:
  |
1 | V1__foo
  |        ^
unexpected end of input
expecting ".sql" or filename

λ> derp "V1__.sql"
1:5:
  |
1 | V1__.sql
  |     ^
unexpected '.'
expecting filename

The error messages could be improved significantly by simply altering the ”version” and ”filename” hints in the source code, e.g. by specifying in human-readable terms what a filename is, and what a version is. Parsec does have the <?> operator, too, but Parsec does not provide this very graphical demonstration of where parsing went wrong, it only reports the label itself, and, IIRC, the Char offset in the input String!

Another advantage of Megaparsec is native Text support; I don't find that it's particularly useful in this usecase, since FilePath is aliased to String.

Simon Shine • Sep 17 '20 • Edited

Excellent!

This is actually a good use-case for regular expressions. In Haskell you have regex-applicative which is also a parser combinator library, except it is not monadic, and for that reason is restricted to regular languages.

Here is how a parser might look like:

{-# LANGUAGE RecordWildCards #-}

module MigratumRegex
  ( MigrationFile
  , prettyMigrationFile
  , parseMigrationFile
  ) where

import Control.Applicative (some)
import Data.Char (isAlphaNum)
import Data.Version (Version, makeVersion, showVersion)

import Text.Regex.Applicative (RE, sym, psym, string, (=~))
import Text.Regex.Applicative.Common (decimal)

data MigrationFile = MigrationFile
  { mfVersion :: Version
  , mfName    :: FilePath
  } deriving (Eq, Show)

type Parser a = RE Char a

prettyMigrationFile :: MigrationFile -> String
prettyMigrationFile MigrationFile{..} =
  "V" <> showVersion mfVersion <> "__" <> mfName <> ".sql"

parseMigrationFile :: String -> Maybe MigrationFile
parseMigrationFile = (=~ migrationFile)

-- V<version number>__<file name>.sql
migrationFile :: Parser MigrationFile
migrationFile =
  MigrationFile
    <$> (sym 'V' *> version)
    <*> (string "__" *> filename <* string ".sql")

version :: Parser Version
version = fmap (makeVersion . pure) decimal

filename :: Parser FilePath
filename = some (psym isAlphaNum)

And here it is in action:

λ> prettyMigrationFile <$> parseMigrationFile "V123__hello.sql"
Just "V123__hello.sql"

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Haskell - Enforcing Naming Convention with Parsec

References

Top comments (2)

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