{-
The parser uses a separate lexer for two reasons:

1. sql syntax is very awkward to parse, the separate lexer makes it
easier to handle this in most places (in some places it makes it
harder or impossible, the fix is to switch to something better than
parsec)

2. using a separate lexer gives a huge speed boost because it reduces
backtracking. (We could get this by making the parsing code a lot more
complex also.)

3. we can test the lexer relatively exhaustively, then even when we
don't do nearly as comprehensive testing on the syntax level, we still
have a relatively high assurance of the low level of bugs. This is
much more difficult to get parity with when testing the syntax parser
directly without the separately testing lexing stage.

TODO:

make the tokenswill print more dialect accurate. Maybe add symbol
  chars and identifier chars to the dialect definition and use them from
  here

start adding negative / different parse dialect tests

add token tables and tests for oracle, sql server
review existing tables

look for refactoring opportunities, especially the token
generation tables in the tests

do some user documentation on lexing, and lexing/dialects

start thinking about a more separated design for the dialect handling

lexing tests are starting to take a really long time, so split the
tests so it is much easier to run all the tests except the lexing
tests which only need to be run when working on the lexer (which
should be relatively uncommon), or doing a commit or finishing off a
series of commits,

start writing the error message tests:
  generate/write a large number of syntax errors
  create a table with the source and the error message
  try to compare some different versions of code to compare the
    quality of the error messages by hand

  get this checked in so improvements and regressions in the error
    message quality can be tracked a little more easily (although it will
    still be manual)

try again to add annotation to the ast

-}

-- | Lexer for SQL.
{-# LANGUAGE TupleSections #-}
{-# LANGUAGE OverloadedStrings #-}
module Language.SQL.SimpleSQL.Lex
    (Token(..)
    ,WithPos(..)
    ,lexSQL
    ,prettyToken
    ,prettyTokens
    ,ParseError
    ,prettyError
    ,tokenListWillPrintAndLex
    ,ansi2011
    ) where

import Language.SQL.SimpleSQL.Dialect
    (Dialect(..)
    ,ansi2011
    )

import Text.Megaparsec
    (Parsec
    ,runParser'

    ,ParseErrorBundle(..)
    ,errorBundlePretty

    ,SourcePos(..)
    ,getSourcePos
    ,getOffset
    ,pstateSourcePos
    ,statePosState
    ,mkPos

    ,choice
    ,satisfy
    ,takeWhileP
    ,takeWhile1P
    ,(<?>)
    ,eof
    ,many
    ,try
    ,option
    )
import Text.Megaparsec.Char
    (string
    ,char
    )
import Text.Megaparsec.State (initialState)

import Data.Void (Void)

import Data.Char
    (isAlphaNum
    ,isAlpha
    ,isSpace
    ,isDigit
    )
import Control.Monad (void)
import Data.Text (Text)
import qualified Data.Text as T

------------------------------------------------------------------------------

-- syntax

-- | Represents a lexed token
data Token
    -- | A symbol (in ansi dialect) is one of the following
    --
    -- * multi char symbols <> \<= \>= != ||
    -- * single char symbols: * + -  < >  ^ / %  ~ & | ? ( ) [ ] , ; ( )
    --
    = Symbol Text
    -- | This is an identifier or keyword. The first field is
    -- the quotes used, or nothing if no quotes were used. The quotes
    -- can be " or u& or something dialect specific like []
    | Identifier (Maybe (Text,Text)) Text
    -- | This is a prefixed variable symbol, such as :var, @var or #var
    -- (only :var is used in ansi dialect)
    | PrefixedVariable Char Text
    -- | This is a positional arg identifier e.g. $1
    | PositionalArg Int
    -- | This is a string literal. The first two fields are the --
    -- start and end quotes, which are usually both ', but can be
    -- the character set (one of nNbBxX, or u&, U&), or a dialect
    -- specific string quoting (such as $$ in postgres)
    | SqlString Text Text Text
    -- | A number literal (integral or otherwise), stored in original format
    -- unchanged
    | SqlNumber Text
    -- | Whitespace, one or more of space, tab or newline.
    | Whitespace Text
    -- | A commented line using --, contains every character starting with the
    -- \'--\' and including the terminating newline character if there is one
    -- - this will be missing if the last line in the source is a line comment
    -- with no trailing newline
    | LineComment Text
    -- | A block comment, \/* stuff *\/, includes the comment delimiters
    | BlockComment Text
      deriving (Eq,Show,Ord)

------------------------------------------------------------------------------

-- main api functions

-- | Lex some SQL to a list of tokens.
lexSQL :: Dialect
       -- ^ dialect of SQL to use
       -> Text
       -- ^ filename to use in error messages
       -> Maybe (Int,Int)
       -- ^ line number and column number of the first character
       -- in the source to use in error messages
       -> Text
       -- ^ the SQL source to lex
       -> Either ParseError [WithPos Token]
lexSQL dialect fn p src = myParse fn p (many (sqlToken dialect) <* (eof <?> "")) src

myParse :: Text -> (Maybe (Int,Int)) -> Parser a -> Text -> Either ParseError a
myParse name sp' p s =
        let sp = maybe (1,1) id sp'
            ps = SourcePos (T.unpack name) (mkPos $ fst sp) (mkPos $ snd sp)
            is = (initialState (T.unpack name) s)
            sps = (statePosState is) {pstateSourcePos = ps}
            is' = is {statePosState = sps}
        in snd $ runParser' p is'

prettyError :: ParseError -> Text
prettyError = T.pack . errorBundlePretty

------------------------------------------------------------------------------

-- parsing boilerplate

type ParseError = ParseErrorBundle Text Void

type Parser = Parsec Void Text

-- | Positional information added to tokens to preserve source positions
-- for the parser
data WithPos a = WithPos
  { startPos :: SourcePos
  , endPos :: SourcePos
  , tokenLength :: Int
  , tokenVal :: a
  } deriving (Eq, Ord, Show)

------------------------------------------------------------------------------

-- pretty print

-- | Pretty printing, if you lex a bunch of tokens, then pretty
-- print them, should should get back exactly the same string
prettyToken :: Dialect -> Token -> Text
prettyToken _ (Symbol s) = s
prettyToken _ (Identifier Nothing t) = t
prettyToken _ (Identifier (Just (q1,q2)) t) = q1 <> t <> q2
prettyToken _ (PrefixedVariable c p) = T.cons c p
prettyToken _ (PositionalArg p) = T.cons '$' $ T.pack $ show p
prettyToken _ (SqlString s e t) = s <> t <> e
prettyToken _ (SqlNumber r) = r
prettyToken _ (Whitespace t) = t
prettyToken _ (LineComment l) = l
prettyToken _ (BlockComment c) = c

prettyTokens :: Dialect -> [Token] -> Text
prettyTokens d ts = T.concat $ map (prettyToken d) ts

------------------------------------------------------------------------------

-- token parsers

-- | parser for a sql token
sqlToken :: Dialect -> Parser (WithPos Token)
sqlToken d = do
    -- possibly there's a more efficient way of doing the source positions?
    sp <- getSourcePos
    off <- getOffset
    t <- choice
         [sqlString d
         ,identifier d
         ,lineComment d
         ,blockComment d
         ,sqlNumber d
         ,positionalArg d
         --,dontParseEndBlockComment d
         ,prefixedVariable d
         ,symbol d
         ,sqlWhitespace d]
    off1 <- getOffset
    ep <- getSourcePos
    pure $ WithPos sp ep (off1 - off) t

--------------------------------------

sqlString :: Dialect -> Parser Token
sqlString _d =
    SqlString "'" "'" <$> (char_ '\'' *> takeWhileP (Just "non quote char") (/= '\'') <* char_ '\'')

{-
Parse a SQL string. Examples:

'basic string'
'string with '' a quote'
n'international text'
b'binary string'
x'hexidecimal string'
-}

{-
sqlString :: Dialect -> Parser Token
sqlString d = dollarString <|> csString <|> normalString
  where
    dollarString = do
        guard $ diDollarString d
        -- use try because of ambiguity with symbols and with
        -- positional arg
        delim <- (\x -> concat ["$",x,"$"])
                 <$> try (char '$' *> option "" identifierString <* char '$')
        SqlString delim delim  <$> manyTill anyChar (try $ string delim)
    normalString = SqlString "'" "'" <$> (char '\'' *> normalStringSuffix False "")
    normalStringSuffix allowBackslash t = do
        s <- takeTill $ if allowBackslash
                        then (`elem` "'\\")
                        else (== '\'')
        -- deal with '' or \' as literal quote character
        choice [do
                ctu <- choice ["''" <$ try (string "''")
                              ,"\\'" <$ string "\\'"
                              ,"\\" <$ char '\\']
                normalStringSuffix allowBackslash $ concat [t,s,ctu]
               ,concat [t,s] <$ char '\'']
    -- try is used to to avoid conflicts with
    -- identifiers which can start with n,b,x,u
    -- once we read the quote type and the starting '
    -- then we commit to a string
    -- it's possible that this will reject some valid syntax
    -- but only pathalogical stuff, and I think the improved
    -- error messages and user predictability make it a good
    -- pragmatic choice
    csString
      | diEString d =
        choice [SqlString <$> try (string "e'" <|> string "E'")
                          <*> return "'" <*> normalStringSuffix True ""
               ,csString']
      | otherwise = csString'
    csString' = SqlString
                <$> try cs
                <*> return "'"
                <*> normalStringSuffix False ""
    csPrefixes = "nNbBxX"
    cs = choice $ (map (\x -> string ([x] ++ "'")) csPrefixes)
                  ++ [string "u&'"
                     ,string "U&'"]
-}

--------------------------------------

-- TODO: this reconstitutes the string from bits, instead of lifting
-- it in one piece from the source. This is a performance issue, not
-- sure if it will be significant. The same comment applies to most of
-- the other parsers
identifier :: Dialect -> Parser Token
identifier d = Identifier Nothing <$> identifierString d

identifierString :: Dialect -> Parser Text
identifierString _ = (do
    c <- satisfy isFirstLetter
    choice
        [T.cons c <$> (takeWhileP (Just "identifier char") isNonFirstLetter)
        ,pure $ T.singleton c]) <?> "identifier"
  where
     isFirstLetter c = c == '_' || isAlpha c
     isNonFirstLetter c = c == '_' || isAlphaNum c

{-
Parses identifiers:

simple_identifier_23
u&"unicode quoted identifier"
"quoted identifier"
"quoted identifier "" with double quote char"
`mysql quoted identifier`
-}

{-
identifier :: Dialect -> Parser Token
identifier d =
    choice
    [quotedIden
    ,unicodeQuotedIden
    ,regularIden
    ,guard (diBackquotedIden d) >> mySqlQuotedIden
    ,guard (diSquareBracketQuotedIden d) >> sqlServerQuotedIden
    ]
  where
    regularIden = Identifier Nothing <$> identifierString
    quotedIden = Identifier (Just ("\"","\"")) <$> qidenPart
    mySqlQuotedIden = Identifier (Just ("`","`"))
                      <$> (char '`' *> takeWhile1 (/='`') <* char '`')
    sqlServerQuotedIden = Identifier (Just ("[","]"))
                          <$> (char '[' *> takeWhile1 (`notElem` "[]") <* char ']')
    -- try is used here to avoid a conflict with identifiers
    -- and quoted strings which also start with a 'u'
    unicodeQuotedIden = Identifier
                        <$> (f <$> try (oneOf "uU" <* string "&"))
                        <*> qidenPart
      where f x = Just (x: "&\"", "\"")
    qidenPart = char '"' *> qidenSuffix ""
    qidenSuffix t = do
        s <- takeTill (=='"')
        void $ char '"'
        -- deal with "" as literal double quote character
        choice [do
                void $ char '"'
                qidenSuffix $ concat [t,s,"\"\""]
               ,return $ concat [t,s]]


-- This parses a valid identifier without quotes.

identifierString :: Parser String
identifierString =
    startsWith (\c -> c == '_' || isAlpha c) isIdentifierChar

-- this can be moved to the dialect at some point

isIdentifierChar :: Char -> Bool
isIdentifierChar c = c == '_' || isAlphaNum c

-}

--------------------------------------

{-
I think it's always faster to use a string locally created in the parser code,
than to use one taken from the parsed source, unless you take it without modifying it,
the example here is using -- and \n. this won't be needed in this case if can work out
how to lift the entire comment as a single string from the source.

this concept does apply to things like symbols
-}

lineComment :: Dialect -> Parser Token
lineComment _ = do
    try (string_ "--") <?> ""
    rest <- takeWhileP (Just "non newline character") (/='\n')
    -- can you optionally read the \n to terminate the takewhilep without reparsing it?
    suf <- option "" ("\n" <$ char_ '\n')
    pure $ LineComment $ T.concat ["--", rest, suf]

{-lineComment :: Dialect -> Parser Token
lineComment _ =
    (\s -> LineComment $ concat ["--",s]) <$>
    -- try is used here in case we see a - symbol
    -- once we read two -- then we commit to the comment token
    (try (string "--") *> (
        -- todo: there must be a better way to do this
     conc <$> manyTill anyChar (lookAhead lineCommentEnd) <*> lineCommentEnd))
  where
    conc a Nothing = a
    conc a (Just b) = a ++ b
    lineCommentEnd =
        Just "\n" <$ char '\n'
        <|> Nothing <$ eof-}

--------------------------------------

blockComment :: Dialect -> Parser Token
blockComment _ = (do
    try $ string_ "/*"
    BlockComment . T.concat . ("/*":) <$> more) <?> ""
  where
    more = choice
        [["*/"] <$ try (string_ "*/")
        ,char_ '*' *> (("*":) <$> more)
        ,(:) <$> takeWhile1P (Just "non comment terminator text") (/= '*') <*> more]



{-
Try is used in the block comment for the two symbol bits because we
want to backtrack if we read the first symbol but the second symbol
isn't there.
-}
{-
blockComment :: Dialect -> Parser Token
blockComment _ =
    (\s -> BlockComment $ concat ["/*",s]) <$>
    (try (string "/*") *> commentSuffix 0)
  where
    commentSuffix :: Int -> Parser String
    commentSuffix n = do
      -- read until a possible end comment or nested comment
      x <- takeWhile (\e -> e /= '/' && e /= '*')
      choice [-- close comment: if the nesting is 0, done
              -- otherwise recurse on commentSuffix
              try (string "*/") *> let t = concat [x,"*/"]
                                   in if n == 0
                                      then return t
                                      else (\s -> concat [t,s]) <$> commentSuffix (n - 1)
              -- nested comment, recurse
             ,try (string "/*") *> ((\s -> concat [x,"/*",s]) <$> commentSuffix (n + 1))
              -- not an end comment or nested comment, continue
             ,(\c s -> x ++ [c] ++ s) <$> anyChar <*> commentSuffix n]
-}

{-
This is to improve user experience: provide an error if we see */
outside a comment. This could potentially break postgres ops with */
in them (which is a stupid thing to do). In other cases, the user
should write * / instead (I can't think of any cases when this would
be valid syntax though).
-}
{-
dontParseEndBlockComment :: Dialect -> Parser Token
dontParseEndBlockComment _ =
    -- don't use try, then it should commit to the error
    try (string "*/") *> fail "comment end without comment start"
-}
--------------------------------------

sqlNumber :: Dialect -> Parser Token
sqlNumber _ =
    SqlNumber <$> digits


digits :: Parser Text
digits = takeWhile1P (Just "digit") isDigit


{-
numbers

digits
digits.[digits][e[+-]digits]
[digits].digits[e[+-]digits]
digitse[+-]digits

where digits is one or more decimal digits (0 through 9). At least one
digit must be before or after the decimal point, if one is used. At
least one digit must follow the exponent marker (e), if one is
present. There cannot be any spaces or other characters embedded in
the constant. Note that any leading plus or minus sign is not actually
considered part of the constant; it is an operator applied to the
constant.
-}

{-
sqlNumber :: Dialect -> Parser Token
sqlNumber d =
    SqlNumber <$> completeNumber
    -- this is for definitely avoiding possibly ambiguous source
    <* choice [-- special case to allow e.g. 1..2
               guard (diPostgresSymbols d)
               *> (void $ lookAhead $ try $ string "..")
                  <|> void (notFollowedBy (oneOf "eE."))
              ,notFollowedBy (oneOf "eE.")
              ]
  where
    completeNumber =
      (int <??> (pp dot <??.> pp int)
      -- try is used in case we read a dot
      -- and it isn't part of a number
      -- if there are any following digits, then we commit
      -- to it being a number and not something else
      <|> try ((++) <$> dot <*> int))
      <??> pp expon

    int = many1 digit
    -- make sure we don't parse two adjacent dots in a number
    -- special case for postgresql, we backtrack if we see two adjacent dots
    -- to parse 1..2, but in other dialects we commit to the failure
    dot = let p = string "." <* notFollowedBy (char '.')
          in if diPostgresSymbols d
             then try p
             else p
    expon = (:) <$> oneOf "eE" <*> sInt
    sInt = (++) <$> option "" (string "+" <|> string "-") <*> int
    pp = (<$$> (++))
-}

--------------------------------------

positionalArg :: Dialect -> Parser Token
positionalArg _ = PositionalArg <$> (char_ '$' *> (read . T.unpack <$> digits))

{-
positionalArg :: Dialect -> Parser Token
positionalArg d =
  guard (diPositionalArg d) >>
  -- use try to avoid ambiguities with other syntax which starts with dollar
  PositionalArg <$> try (char '$' *> (read <$> many1 digit))
-}

--------------------------------------

-- todo: I think the try here should read a prefix char, then a single valid
-- identifier char, then commit
prefixedVariable :: Dialect -> Parser Token
prefixedVariable d = try $ choice
    [PrefixedVariable <$> (':' <$ char_ ':') <*> identifierString d
    ]


-- use try because : and @ can be part of other things also

{-
prefixedVariable :: Dialect -> Parser Token
prefixedVariable  d = try $ choice
    [PrefixedVariable <$> char ':' <*> identifierString
    ,guard (diAtIdentifier d) >>
     PrefixedVariable <$> char '@' <*> identifierString
    ,guard (diHashIdentifier d) >>
     PrefixedVariable <$> char '#' <*> identifierString
    ]
-}

--------------------------------------

symbol :: Dialect -> Parser Token
symbol _ =
    Symbol <$> choice
    [try $ choice $ map (\a -> string a) multiCharSymbols
    ,T.singleton <$> satisfy (`elem` singleLetterSymbol)
    ]
  where
    singleLetterSymbol = "(),-+*/<>=." :: String
    multiCharSymbols = ["!=", "<>", ">=", "<=", "||"]


{-
Symbols

A symbol is an operator, or one of the misc symbols which include:
. .. := : :: ( ) ? ; , { } (for odbc)

The postgresql operator syntax allows a huge range of operators
compared with ansi and other dialects
-}
{-
symbol :: Dialect -> Parser Token
symbol d  = Symbol <$> choice (concat
   [dots
   ,if diPostgresSymbols d
    then postgresExtraSymbols
    else []
   ,miscSymbol
   ,if diOdbc d then odbcSymbol else []
   ,if diPostgresSymbols d
    then generalizedPostgresqlOperator
    else basicAnsiOps
   ])
 where
   dots = [many1 (char '.')]
   odbcSymbol = [string "{", string "}"]
   postgresExtraSymbols =
       [try (string ":=")
        -- parse :: and : and avoid allowing ::: or more
       ,try (string "::" <* notFollowedBy (char ':'))
       ,try (string ":" <* notFollowedBy (char ':'))]
   miscSymbol = map (string . (:[])) $
                case () of
                    _ | diSqlServerSymbols d -> ",;():?"
                      | diPostgresSymbols d -> "[],;()"
                      | otherwise -> "[],;():?"

{-
try is used because most of the first characters of the two character
symbols can also be part of a single character symbol
-}

   basicAnsiOps = map (try . string) [">=","<=","!=","<>"]
                  ++ map (string . (:[])) "+-^*/%~&<>="
                  ++ pipes
   pipes = -- what about using many1 (char '|'), then it will
           -- fail in the parser? Not sure exactly how
           -- standalone the lexer should be
           [char '|' *>
            choice ["||" <$ char '|' <* notFollowedBy (char '|')
                   ,return "|"]]

{-
postgresql generalized operators

this includes the custom operators that postgres supports,
plus all the standard operators which could be custom operators
according to their grammar

rules

An operator name is a sequence of up to NAMEDATALEN-1 (63 by default) characters from the following list:

+ - * / < > = ~ ! @ # % ^ & | ` ?

There are a few restrictions on operator names, however:
-- and /* cannot appear anywhere in an operator name, since they will be taken as the start of a comment.

A multiple-character operator name cannot end in + or -, unless the name also contains at least one of these characters:

~ ! @ # % ^ & | ` ?

which allows the last character of a multi character symbol to be + or
-
-}

generalizedPostgresqlOperator :: [Parser String]
generalizedPostgresqlOperator = [singlePlusMinus,opMoreChars]
  where
    allOpSymbols = "+-*/<>=~!@#%^&|`?"
    -- these are the symbols when if part of a multi character
    -- operator permit the operator to end with a + or - symbol
    exceptionOpSymbols = "~!@#%^&|`?"

    -- special case for parsing a single + or - symbol
    singlePlusMinus = try $ do
      c <- oneOf "+-"
      notFollowedBy $ oneOf allOpSymbols
      return [c]

    -- this is used when we are parsing a potentially multi symbol
    -- operator and we have alread seen one of the 'exception chars'
    -- and so we can end with a + or -
    moreOpCharsException = do
       c <- oneOf (filter (`notElem` "-/*") allOpSymbols)
            -- make sure we don't parse a comment starting token
            -- as part of an operator
            <|> try (char '/' <* notFollowedBy (char '*'))
            <|> try (char '-' <* notFollowedBy (char '-'))
            -- and make sure we don't parse a block comment end
            -- as part of another symbol
            <|> try (char '*' <* notFollowedBy (char '/'))
       (c:) <$> option [] moreOpCharsException

    opMoreChars = choice
       [-- parse an exception char, now we can finish with a + -
        (:)
        <$> oneOf exceptionOpSymbols
        <*> option [] moreOpCharsException
       ,(:)
        <$> (-- parse +, make sure it isn't the last symbol
             try (char '+' <* lookAhead (oneOf allOpSymbols))
             <|> -- parse -, make sure it isn't the last symbol
                 -- or the start of a -- comment
             try (char '-'
                  <* notFollowedBy (char '-')
                  <* lookAhead (oneOf allOpSymbols))
             <|> -- parse / check it isn't the start of a /* comment
             try (char '/' <* notFollowedBy (char '*'))
             <|> -- make sure we don't parse */ as part of a symbol
             try (char '*' <* notFollowedBy (char '/'))
             <|> -- any other ansi operator symbol
             oneOf "<>=")
        <*> option [] opMoreChars
       ]
-}

--------------------------------------

sqlWhitespace :: Dialect -> Parser Token
sqlWhitespace _ = Whitespace <$> takeWhile1P (Just "whitespace") isSpace <?> ""

--sqlWhitespace :: Dialect -> Parser Token
--sqlWhitespace _ = Whitespace <$> many1 (satisfy isSpace)

----------------------------------------------------------------------------

-- parser helpers

char_ :: Char -> Parser ()
char_ = void . char

string_ :: Text -> Parser ()
string_ = void . string

{-
startsWith :: (Char -> Bool) -> (Char -> Bool) -> Parser String
startsWith p ps = do
  c <- satisfy p
  choice [(:) c <$> (takeWhile1 ps)
         ,return [c]]

takeWhile1 :: (Char -> Bool) -> Parser String
takeWhile1 p = many1 (satisfy p)

takeWhile :: (Char -> Bool) -> Parser String
takeWhile p = many (satisfy p)

takeTill :: (Char -> Bool) -> Parser String
takeTill p = manyTill anyChar (peekSatisfy p)

peekSatisfy :: (Char -> Bool) -> Parser ()
peekSatisfy p = void $ lookAhead (satisfy p)
-}

----------------------------------------------------------------------------


{-
This utility function will accurately report if the two tokens are
pretty printed, if they should lex back to the same two tokens. This
function is used in testing (and can be used in other places), and
must not be implemented by actually trying to print both tokens and
then lex them back from a single string (because then we would have
the risk of thinking two tokens cannot be together when there is bug
in the lexer, which the testing is supposed to find).

maybe do some quick checking to make sure this function only gives
true negatives: check pairs which return false actually fail to lex or
give different symbols in return: could use quickcheck for this

a good sanity test for this function is to change it to always return
true, then check that the automated tests return the same number of
successes. I don't think it succeeds this test at the moment
-}

-- | Utility function to tell you if a list of tokens
-- will pretty print then lex back to the same set of tokens.
-- Used internally, might be useful for generating SQL via lexical tokens.
tokenListWillPrintAndLex :: Dialect -> [Token] -> Bool
tokenListWillPrintAndLex = undefined
{-tokenListWillPrintAndLex _ [] = True
tokenListWillPrintAndLex _ [_] = True
tokenListWillPrintAndLex d (a:b:xs) =
    tokensWillPrintAndLex d a b && tokenListWillPrintAndLex d (b:xs)

tokensWillPrintAndLex :: Dialect -> Token -> Token -> Bool
tokensWillPrintAndLex d a b

{-
a : followed by an identifier character will look like a host param
followed by = or : makes a different symbol
-}

    | Symbol ":" <- a
    , checkFirstBChar (\x -> isIdentifierChar x || x `elem` ":=") = False

{-
two symbols next to eachother will fail if the symbols can combine and
(possibly just the prefix) look like a different symbol
-}

    | diPostgresSymbols d
    , Symbol a' <- a
    , Symbol b' <- b
    , b' `notElem` ["+", "-"] || or (map (`elem` a') "~!@#%^&|`?") = False

{-
check two adjacent symbols in non postgres where the combination
possibilities are much more limited. This is ansi behaviour, it might
be different when the other dialects are done properly
-}

   | Symbol a' <- a
   , Symbol b' <- b
   , (a',b') `elem` [("<",">")
                    ,("<","=")
                    ,(">","=")
                    ,("!","=")
                    ,("|","|")
                    ,("||","|")
                    ,("|","||")
                    ,("||","||")
                    ,("<",">=")
                    ] = False

-- two whitespaces will be combined

   | Whitespace {} <- a
   , Whitespace {} <- b = False

-- line comment without a newline at the end will eat the next token

   | LineComment {} <- a
   , checkLastAChar (/='\n') = False

{-
check the last character of the first token and the first character of
the second token forming a comment start or end symbol
-}

   | let f '-' '-' = True
         f '/' '*' = True
         f '*' '/' = True
         f _ _ = False
     in checkBorderChars f = False

{-
a symbol will absorb a following .
TODO: not 100% on this always being bad
-}

   | Symbol {} <- a
   , checkFirstBChar (=='.') = False

-- cannot follow a symbol ending in : with another token starting with :

   | let f ':' ':' = True
         f _ _ = False
     in checkBorderChars f = False

-- unquoted identifier followed by an identifier letter

   | Identifier Nothing _ <- a
   , checkFirstBChar isIdentifierChar = False

-- a quoted identifier using ", followed by a " will fail

   | Identifier (Just (_,"\"")) _ <- a
   , checkFirstBChar (=='"') = False

-- prefixed variable followed by an identifier char will be absorbed

   | PrefixedVariable {} <- a
   , checkFirstBChar isIdentifierChar = False

-- a positional arg will absorb a following digit

   | PositionalArg {} <- a
   , checkFirstBChar isDigit = False

-- a string ending with ' followed by a token starting with ' will be absorbed

   | SqlString _ "'" _ <- a
   , checkFirstBChar (=='\'') = False

-- a number followed by a . will fail or be absorbed

   | SqlNumber {} <- a
   , checkFirstBChar (=='.') = False

-- a number followed by an e or E will fail or be absorbed

   | SqlNumber {} <- a
   , checkFirstBChar (\x -> x =='e' || x == 'E') = False

-- two numbers next to eachother will fail or be absorbed

   | SqlNumber {} <- a
   , SqlNumber {} <- b = False


   | otherwise = True

  where
    prettya = prettyToken d a
    prettyb = prettyToken d b
    -- helper function to run a predicate on the
    -- last character of the first token and the first
    -- character of the second token
    checkBorderChars f
        | (_:_) <- prettya
        , (fb:_) <- prettyb
        , la <- last prettya
        = f la fb
    checkBorderChars _ = False
    checkFirstBChar f = case prettyb of
                          (b':_) -> f b'
                          _ -> False
    checkLastAChar f = case prettya of
                          (_:_) -> f $ last prettya
                          _ -> False

-}