Superior string spaghetti with PureScript

Lightning talk I gave at HaskellX 2018

Transcript

1. Superior string spaghetti with PureScript
   Justin Woo
   October 12 2018
   Justin Woo Superior string spaghetti with PureScript October 12 2018 1 / 17
   

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2. Problem: Untyped Parameterized SQL Queries
   We want to work with parameterized SQL queries, but they’re always
   untyped
   thing =
   queryDB """
   select name, count from mytable
   where name = $name and count = $count
   """
   { "$name": "Bill"
   , "$count": 10
   }
   But we know statically what the query string is here!
   What if we could use it at the type level and extract a type from it?
   Justin Woo Superior string spaghetti with PureScript October 12 2018 2 / 17
   

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3. What if you could Cons Symbol like Lists?
   In PureScript 0.12, we can!
   “ABC” -> “A” “BC”
   class Cons (head :: Symbol) (tail :: Symbol) (symbol :: Symbol
   | head tail -> symbol, symbol -> head tail
   Now we can parse Symbols!
   class ParseParamName (x :: Symbol) (xs :: Symbol)
   (acc :: Symbol) (out :: Symbol)
   | x xs -> acc out
   Justin Woo Superior string spaghetti with PureScript October 12 2018 3 / 17
   

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4. The problem with normal instance matching
   When parsing, we need to work with both the head and the tail
   E.g. when parsing a param name until a space or end of the string
   class ParseParamName (x :: Symbol) (xs :: Symbol)
   (acc :: Symbol) (out :: Symbol) | -- ...
   -- invalid overloading instances!
   instance endRParseParamName :: ( Symbol.Append acc x out ) =>
   ParseParamName x "" acc out
   instance spaceParseParamName :: ParseParamName " " xs out out
   Justin Woo Superior string spaghetti with PureScript October 12 2018 4 / 17
   

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5. Instance chains at work
   In PureScript 0.12, we have instance chains (groups)
   First come, first served, with regular fundep-based instance matching (not
   constraints/guards)
   instance endRParseParamName ::
   ( Symbol.Append acc x out
   ) => ParseParamName x "" acc out
   -- note the else here:
   else instance spaceParseParamName ::
   ParseParamName " " xs out out
   Instance chain (groups) can only be implemented within a module, which is
   fine
   Justin Woo Superior string spaghetti with PureScript October 12 2018 5 / 17
   

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6. How do these solve our problem?
   Now we can write a parser at the type level, and we can synthesize row
   types as usual:
   getEm :: forall a b
   . AllowedParamType a => AllowedParamType b
   => DBConnection
   -> { "$name" :: a
   , "$count" :: b
   }
   -> Aff Foreign
   getEm db = J.queryDB db $ SProxy :: SProxy """
   select name, count from mytable
   where name = $name and count = $count
   """
   Justin Woo Superior string spaghetti with PureScript October 12 2018 6 / 17
   

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7. Top level function
   queryDB :: forall query params
   . IsSymbol query => ExtractParams query params
   => SQLite3.DBConnection
   -> SProxy query
   -> { | params }
   -> Aff Foreign
   class ExtractParams (query :: Symbol) (params :: # Type)
   | query -> params
   instance extractParams ::
   ( Symbol.Cons x xs query
   , ExtractParamsParse x xs params
   ) => ExtractParams query params
   Justin Woo Superior string spaghetti with PureScript October 12 2018 7 / 17
   

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8. Parsing and Extracting Params
   class ExtractParamsParse
   (x :: Symbol) -- current character
   (xs :: Symbol) -- tail
   (params :: # Type) -- row type of parsed parameters
   | x xs -> params -- fundeps, params are determined
   -- base case, no more to extract at end of string:
   instance endExtractParamsParse :: ExtractParamsParse x "" ()
   Justin Woo Superior string spaghetti with PureScript October 12 2018 8 / 17
   

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9. On “$”, parse out the parameter name and add it to our record
   else instance paramExtractParams ::
   ( Symbol.Cons y ys xs
   , ParseParamName y ys "$" out
   , Symbol.Cons z zs ys
   , Row.Cons out ty row row
   , AllowedParamType ty
   , ExtractParamsParse z zs row
   ) => ExtractParamsParse "$" xs row
   Otherwise, continue
   else instance nExtractParams ::
   ( Symbol.Cons y ys xs
   , ExtractParamsParse y ys row
   ) => ExtractParamsParse x xs row
   Justin Woo Superior string spaghetti with PureScript October 12 2018 9 / 17
   

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10. Row.Cons?
    Remember that record types are parameterized by row type in PureScript
    data Record :: # Type -> Type
    type MyRecord = { a :: Int, b :: String }
    ~ Record ( a :: Int, b :: String )
    So it makes sense we can use RowCons to add to it
    class Cons (label :: Symbol) (a :: Type)
    (tail :: # Type) (row :: # Type)
    | label a tail -> row, label row -> a tail
    Justin Woo Superior string spaghetti with PureScript October 12 2018 10 / 17
    

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11. Figure 1:
    Justin Woo Superior string spaghetti with PureScript October 12 2018 11 / 17
    

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12. Back to the top
    Hopefully now this all makes sense:
    queryDB :: forall query params
    . IsSymbol query => ExtractParams query params
    => SQLite3.DBConnection
    -> SProxy query
    -> { | params }
    -> Aff Foreign
    Justin Woo Superior string spaghetti with PureScript October 12 2018 12 / 17
    

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13. getEm :: forall a b
    . AllowedParamType a => AllowedParamType b
    => DBConnection
    -> { "$name" :: a, "$count" :: b }
    -> Aff Foreign
    getEm db = J.queryDB db $ SProxy :: SProxy """
    select name, count from mytable
    where name = $name and count = $count
    """
    Justin Woo Superior string spaghetti with PureScript October 12 2018 13 / 17
    

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14. Available as a library
    https://github.com/justinwoo/purescript-jajanmen
    Justin Woo Superior string spaghetti with PureScript October 12 2018 14 / 17
    

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15. Conclusion
    With PureScript 0.12, we can. . .
    Extract a lot of information from Symbols
    Use instance chains to safely use overlapping instances
    Use existing techniques from 0.11.x to synthesize types using the
    extracted information from Symbols
    Justin Woo Superior string spaghetti with PureScript October 12 2018 15 / 17
    

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16. Thanks
    More detailed post here
    https://github.com/justinwoo/my-blog-posts#
    well-typed-parameterized-sqlite-parameters-with-purescript
    Csongor Kiss’s post on Symbol.Cons and his printf library:
    http://kcsongor.github.io/purescript-safe-printf/
    Twitter: @jusrin00
    Justin Woo Superior string spaghetti with PureScript October 12 2018 16 / 17
    

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17. Figure 3: Your face on PureScript
    Justin Woo Superior string spaghetti with PureScript October 12 2018 17 / 17
    

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