relational-query-0.9.0.0: src/Database/Relational/Query/Projectable.hs

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE OverloadedStrings #-}

-- |
-- Module      : Database.Relational.Query.Projectable
-- Copyright   : 2013-2017 Kei Hibino
-- License     : BSD3
--
-- Maintainer  : ex8k.hibino@gmail.com
-- Stability   : experimental
-- Portability : unknown
--
-- This module defines operators on various polymorphic projections.
module Database.Relational.Query.Projectable (
  -- * Projectable from SQL strings
  SqlProjectable (unsafeProjectSqlTerms), unsafeProjectSql',
  unsafeProjectSql,

  -- * Projections of values
  value,
  valueTrue, valueFalse,
  values,
  unsafeValueNull,

  -- * Placeholders
  PlaceHolders, unsafeAddPlaceHolders, unsafePlaceHolders,
  placeholder', placeholder, unitPlaceHolder,

  -- * Projectable into SQL strings
  ProjectableShowSql (unsafeShowSql'), unsafeShowSql,

  -- * Operators
  (.=.), (.<.), (.<=.), (.>.), (.>=.), (.<>.),

  and', or', in',

  (.||.), (?||?), like, likeMaybe, like', likeMaybe',
  (.+.), (.-.), (.*.), (./.),
  (?+?), (?-?), (?*?), (?/?),

  isNothing, isJust, fromMaybe,
  not', exists,

  negate', fromIntegral', showNum,
  negateMaybe, fromIntegralMaybe, showNumMaybe,

  casesOrElse, casesOrElse',
  caseSearch, caseSearchMaybe, case', caseMaybe,

  SqlBinOp, unsafeBinOp, unsafeUniOp,

  -- * Terms for Window function types
  rank, denseRank, rowNumber, percentRank, cumeDist,

  -- * Zipping projections
  projectZip, (><),
  ProjectableIdZip (..),

  -- * 'Maybe' type projecitoins
  ProjectableMaybe (just, flattenMaybe),
  ) where

import Prelude hiding (pi)

import Data.String (IsString)
import Data.Monoid ((<>), mconcat)
import Control.Applicative ((<$>))

import Language.SQL.Keyword (Keyword)
import qualified Language.SQL.Keyword as SQL

import Database.Record
  (PersistableWidth, PersistableRecordWidth, derivedWidth,
   HasColumnConstraint, NotNull)

import Database.Relational.Query.Internal.SQL (StringSQL, stringSQL, showStringSQL)

import Database.Relational.Query.ProjectableClass
  (ProjectableFunctor (..), ProjectableApplicative (..), )
import Database.Relational.Query.Context (Flat, Aggregated, Exists, OverWindow)
import Database.Relational.Query.TupleInstances ()
import Database.Relational.Query.ProjectableClass
  (ShowConstantTermsSQL, showConstantTermsSQL, )
import Database.Relational.Query.Projection
  (Projection, ListProjection)
import qualified Database.Relational.Query.Projection as Projection


-- | Interface to project SQL terms unsafely.
class SqlProjectable p where
  -- | Unsafely project from SQL expression terms.
  unsafeProjectSqlTerms :: [StringSQL] -- ^ SQL expression strings
                        -> p t         -- ^ Result projection object

-- | Unsafely make 'Projection' from SQL terms.
instance SqlProjectable (Projection Flat) where
  unsafeProjectSqlTerms = Projection.unsafeFromSqlTerms

-- | Unsafely make 'Projection' from SQL terms.
instance SqlProjectable (Projection Aggregated) where
  unsafeProjectSqlTerms = Projection.unsafeFromSqlTerms

-- | Unsafely make 'Projection' from SQL terms.
instance SqlProjectable (Projection OverWindow) where
  unsafeProjectSqlTerms = Projection.unsafeFromSqlTerms

class SqlProjectable p => OperatorProjectable p
instance OperatorProjectable (Projection Flat)
instance OperatorProjectable (Projection Aggregated)

-- | Unsafely Project single SQL term.
unsafeProjectSql' :: SqlProjectable p => StringSQL -> p t
unsafeProjectSql' =  unsafeProjectSqlTerms . (:[])

-- | Unsafely Project single SQL string. String interface of 'unsafeProjectSql''.
unsafeProjectSql :: SqlProjectable p => String -> p t
unsafeProjectSql =  unsafeProjectSql' . stringSQL

-- | Polymorphic projection of SQL null value.
unsafeValueNull :: OperatorProjectable p => p (Maybe a)
unsafeValueNull =  unsafeProjectSql "NULL"

-- | Generate polymorphic projection of SQL constant values from Haskell value.
value :: (ShowConstantTermsSQL t, OperatorProjectable p) => t -> p t
value = unsafeProjectSqlTerms . showConstantTermsSQL

-- | Polymorphic proejction of SQL true value.
valueTrue  :: (OperatorProjectable p, ProjectableMaybe p) => p (Maybe Bool)
valueTrue  =  just $ value True

-- | Polymorphic proejction of SQL false value.
valueFalse :: (OperatorProjectable p, ProjectableMaybe p) => p (Maybe Bool)
valueFalse =  just $ value False

-- | Polymorphic proejction of SQL set value from Haskell list.
values :: (ShowConstantTermsSQL t, OperatorProjectable p) => [t] -> ListProjection p t
values =  Projection.list . map value


-- | Interface to get SQL term from projections.
class ProjectableShowSql p where
  -- | Unsafely generate SQL expression term from projection object.
  unsafeShowSql' :: p a       -- ^ Source projection object
                 -> StringSQL -- ^ Result SQL expression string.

-- | Unsafely generate SQL expression string from projection object.
--   String interface of 'unsafeShowSql''.
unsafeShowSql :: ProjectableShowSql p
              => p a    -- ^ Source projection object
              -> String -- ^ Result SQL expression string.
unsafeShowSql =  showStringSQL . unsafeShowSql'

-- | Unsafely get SQL term from 'Proejction'.
instance ProjectableShowSql (Projection c) where
  unsafeShowSql' = Projection.unsafeStringSql


-- | Binary operator type for SQL String.
type SqlBinOp = Keyword -> Keyword -> Keyword

-- | Unsafely make projection unary operator from SQL keyword.
unsafeUniOp :: (ProjectableShowSql p0, SqlProjectable p1)
             => (Keyword -> Keyword) -> p0 a -> p1 b
unsafeUniOp u = unsafeProjectSql' . u . unsafeShowSql'

unsafeFlatUniOp :: (SqlProjectable p, ProjectableShowSql p)
               => Keyword -> p a -> p b
unsafeFlatUniOp kw = unsafeUniOp (SQL.paren . SQL.defineUniOp kw)

-- | Unsafely make projection binary operator from string binary operator.
unsafeBinOp :: (SqlProjectable p, ProjectableShowSql p)
            => SqlBinOp
            -> p a -> p b -> p c
unsafeBinOp op a b = unsafeProjectSql' . SQL.paren $
                     op (unsafeShowSql' a) (unsafeShowSql' b)

-- | Unsafely make compare projection binary operator from string binary operator.
compareBinOp :: (SqlProjectable p, ProjectableShowSql p)
             => SqlBinOp
             -> p a -> p a -> p (Maybe Bool)
compareBinOp =  unsafeBinOp

-- | Unsafely make numrical projection binary operator from string binary operator.
monoBinOp :: (SqlProjectable p, ProjectableShowSql p)
         => SqlBinOp
         -> p a -> p a -> p a
monoBinOp =  unsafeBinOp


-- | Compare operator corresponding SQL /=/ .
(.=.)  :: (OperatorProjectable p, ProjectableShowSql p)
  => p ft -> p ft -> p (Maybe Bool)
(.=.)  =  compareBinOp (SQL..=.)

-- | Compare operator corresponding SQL /</ .
(.<.)  :: (OperatorProjectable p, ProjectableShowSql p)
  => p ft -> p ft -> p (Maybe Bool)
(.<.)  =  compareBinOp (SQL..<.)

-- | Compare operator corresponding SQL /<=/ .
(.<=.)  :: (OperatorProjectable p, ProjectableShowSql p)
  => p ft -> p ft -> p (Maybe Bool)
(.<=.)  =  compareBinOp (SQL..<=.)

-- | Compare operator corresponding SQL />/ .
(.>.)  :: (OperatorProjectable p, ProjectableShowSql p)
  => p ft -> p ft -> p (Maybe Bool)
(.>.)  =  compareBinOp (SQL..>.)

-- | Compare operator corresponding SQL />=/ .
(.>=.)  :: (OperatorProjectable p, ProjectableShowSql p)
  => p ft -> p ft -> p (Maybe Bool)
(.>=.)  =  compareBinOp (SQL..>=.)

-- | Compare operator corresponding SQL /<>/ .
(.<>.) :: (OperatorProjectable p, ProjectableShowSql p)
  => p ft -> p ft -> p (Maybe Bool)
(.<>.) =  compareBinOp (SQL..<>.)

-- | Logical operator corresponding SQL /AND/ .
and' :: (OperatorProjectable p, ProjectableShowSql p)
     => p (Maybe Bool) -> p (Maybe Bool) -> p (Maybe Bool)
and' = monoBinOp SQL.and

-- | Logical operator corresponding SQL /OR/ .
or' :: (OperatorProjectable p, ProjectableShowSql p)
    => p (Maybe Bool) -> p (Maybe Bool) -> p (Maybe Bool)
or'  = monoBinOp SQL.or

-- | Logical operator corresponding SQL /NOT/ .
not' :: (OperatorProjectable p, ProjectableShowSql p)
    => p (Maybe Bool) -> p (Maybe Bool)
not' =  unsafeFlatUniOp SQL.NOT

-- | Logical operator corresponding SQL /EXISTS/ .
exists :: (OperatorProjectable p, ProjectableShowSql p)
       => ListProjection (Projection Exists) r -> p (Maybe Bool)
exists =  unsafeProjectSql' . SQL.paren . SQL.defineUniOp SQL.EXISTS
          . Projection.unsafeStringSqlList unsafeShowSql'

-- | Concatinate operator corresponding SQL /||/ .
(.||.) :: (OperatorProjectable p, ProjectableShowSql p, IsString a)
       => p a -> p a -> p a
(.||.) =  unsafeBinOp (SQL..||.)

-- | Concatinate operator corresponding SQL /||/ . Maybe type version.
(?||?) :: (OperatorProjectable p, ProjectableShowSql p, IsString a)
       => p (Maybe a) -> p (Maybe a) -> p (Maybe a)
(?||?) =  unsafeBinOp (SQL..||.)

unsafeLike :: (OperatorProjectable p, ProjectableShowSql p)
           => p a -> p b -> p (Maybe Bool)
unsafeLike = unsafeBinOp (SQL.defineBinOp SQL.LIKE)

-- | String-compare operator corresponding SQL /LIKE/ .
like' :: (OperatorProjectable p, ProjectableShowSql p, IsString a)
      => p a -> p a -> p (Maybe Bool)
x `like'` y = x `unsafeLike` y

-- | String-compare operator corresponding SQL /LIKE/ .
likeMaybe' :: (OperatorProjectable p, ProjectableShowSql p, IsString a)
           => p (Maybe a) -> p (Maybe a) -> p (Maybe Bool)
x `likeMaybe'` y = x `unsafeLike` y

-- | String-compare operator corresponding SQL /LIKE/ .
like :: (OperatorProjectable p, ProjectableShowSql p, IsString a, ShowConstantTermsSQL a)
       => p a -> a -> p (Maybe Bool)
x `like` a = x `like'` value a

-- | String-compare operator corresponding SQL /LIKE/ . Maybe type version.
likeMaybe :: (OperatorProjectable p, ProjectableShowSql p, IsString a, ShowConstantTermsSQL a)
       => p (Maybe a) -> a -> p (Maybe Bool)
x `likeMaybe` a = x `unsafeLike` value a

-- | Unsafely make number projection binary operator from SQL operator string.
monoBinOp' :: (SqlProjectable p, ProjectableShowSql p)
          => Keyword -> p a -> p a -> p a
monoBinOp' = monoBinOp . SQL.defineBinOp

-- | Number operator corresponding SQL /+/ .
(.+.) :: (OperatorProjectable p, ProjectableShowSql p, Num a)
  => p a -> p a -> p a
(.+.) =  monoBinOp' "+"

-- | Number operator corresponding SQL /-/ .
(.-.) :: (OperatorProjectable p, ProjectableShowSql p, Num a)
  => p a -> p a -> p a
(.-.) =  monoBinOp' "-"

-- | Number operator corresponding SQL /// .
(./.) :: (OperatorProjectable p, ProjectableShowSql p, Num a)
  => p a -> p a -> p a
(./.) =  monoBinOp' "/"

-- | Number operator corresponding SQL /*/ .
(.*.) :: (OperatorProjectable p, ProjectableShowSql p, Num a)
  => p a -> p a -> p a
(.*.) =  monoBinOp' "*"

-- | Number negate uni-operator corresponding SQL /-/.
negate' :: (OperatorProjectable p, ProjectableShowSql p, Num a)
        => p a -> p a
negate' =  unsafeFlatUniOp $ SQL.word "-"

unsafeCastProjectable :: (SqlProjectable p, ProjectableShowSql p)
                           => p a -> p b
unsafeCastProjectable = unsafeProjectSql' . unsafeShowSql'

-- | Number fromIntegral uni-operator.
fromIntegral' :: (SqlProjectable p, ProjectableShowSql p, Integral a, Num b)
              => p a -> p b
fromIntegral' =  unsafeCastProjectable

-- | Unsafely show number into string-like type in projections.
showNum :: (SqlProjectable p, ProjectableShowSql p, Num a, IsString b)
              => p a -> p b
showNum =  unsafeCastProjectable

-- | Number operator corresponding SQL /+/ .
(?+?) :: (OperatorProjectable p, ProjectableShowSql p, Num a)
  => p (Maybe a) -> p (Maybe a) -> p (Maybe a)
(?+?) =  monoBinOp' "+"

-- | Number operator corresponding SQL /-/ .
(?-?) :: (OperatorProjectable p, ProjectableShowSql p, Num a)
  => p (Maybe a) -> p (Maybe a) -> p (Maybe a)
(?-?) =  monoBinOp' "-"

-- | Number operator corresponding SQL /// .
(?/?) :: (OperatorProjectable p, ProjectableShowSql p, Num a)
  => p (Maybe a) -> p (Maybe a) -> p (Maybe a)
(?/?) =  monoBinOp' "/"

-- | Number operator corresponding SQL /*/ .
(?*?) :: (OperatorProjectable p, ProjectableShowSql p, Num a)
  => p (Maybe a) -> p (Maybe a) -> p (Maybe a)
(?*?) =  monoBinOp' "*"

-- | Number negate uni-operator corresponding SQL /-/.
negateMaybe :: (OperatorProjectable p, ProjectableShowSql p, Num a)
            => p (Maybe a) -> p (Maybe a)
negateMaybe =  unsafeFlatUniOp $ SQL.word "-"

-- | Number fromIntegral uni-operator.
fromIntegralMaybe :: (SqlProjectable p, ProjectableShowSql p, Integral a, Num b)
                  => p (Maybe a) -> p (Maybe b)
fromIntegralMaybe =  unsafeCastProjectable

-- | Unsafely show number into string-like type in projections.
showNumMaybe :: (SqlProjectable p, ProjectableShowSql p, Num a, IsString b)
                   => p (Maybe a) -> p (Maybe b)
showNumMaybe = unsafeCastProjectable

whensClause :: (OperatorProjectable p, ProjectableShowSql p)
            => String       -- ^ Error tag
            -> [(p a, p b)] -- ^ Each when clauses
            -> p b          -- ^ Else result projection
            -> Keyword      -- ^ Result projection
whensClause eTag cs0 e = d cs0  where
  d []       = error $ eTag ++ ": Empty when clauses!"
  d cs@(_:_) = mconcat [when' p r | (p, r) <- cs] <> else' <> SQL.END
  when' p r = SQL.WHEN <> unsafeShowSql' p <> SQL.THEN <> unsafeShowSql' r
  else'     = SQL.ELSE <> unsafeShowSql' e

-- | Search case operator correnponding SQL search /CASE/.
--   Like, /CASE WHEN p0 THEN a WHEN p1 THEN b ... ELSE c END/
caseSearch :: (OperatorProjectable p, ProjectableShowSql p)
           => [(p (Maybe Bool), p a)] -- ^ Each when clauses
           -> p a                     -- ^ Else result projection
           -> p a                     -- ^ Result projection
caseSearch cs e = unsafeProjectSql' $ SQL.CASE <> whensClause "caseSearch" cs e

-- | Same as 'caseSearch', but you can write like <when list> `casesOrElse` <else clause>.
casesOrElse :: (OperatorProjectable p, ProjectableShowSql p)
            => [(p (Maybe Bool), p a)] -- ^ Each when clauses
            -> p a                     -- ^ Else result projection
            -> p a                     -- ^ Result projection
casesOrElse = caseSearch

-- | Null default version of 'caseSearch'.
caseSearchMaybe :: (OperatorProjectable p, ProjectableShowSql p)
                => [(p (Maybe Bool), p (Maybe a))] -- ^ Each when clauses
                -> p (Maybe a)                     -- ^ Result projection
caseSearchMaybe cs = caseSearch cs unsafeValueNull

-- | Simple case operator correnponding SQL simple /CASE/.
--   Like, /CASE x WHEN v THEN a WHEN w THEN b ... ELSE c END/
case' :: (OperatorProjectable p, ProjectableShowSql p)
      => p a          -- ^ Projection value to match
      -> [(p a, p b)] -- ^ Each when clauses
      -> p b          -- ^ Else result projection
      -> p b          -- ^ Result projection
case' v cs e = unsafeProjectSql' $ SQL.CASE <> unsafeShowSql' v <> whensClause "case'" cs e

-- | Uncurry version of 'case'', and you can write like ... `casesOrElse'` <else clause>.
casesOrElse' :: (OperatorProjectable p, ProjectableShowSql p)
             => (p a, [(p a, p b)]) -- ^ Projection value to match and each when clauses list
             -> p b                 -- ^ Else result projection
             -> p b                 -- ^ Result projection
casesOrElse' =  uncurry case'

-- | Null default version of 'case''.
caseMaybe :: (OperatorProjectable p, ProjectableShowSql p, ProjectableMaybe p)
          => p a                  -- ^ Projection value to match
          -> [(p a, p (Maybe b))] -- ^ Each when clauses
          -> p (Maybe b)          -- ^ Result projection
caseMaybe v cs = case' v cs unsafeValueNull

-- | Binary operator corresponding SQL /IN/ .
in' :: (OperatorProjectable p, ProjectableShowSql p)
    => p t -> ListProjection p t -> p (Maybe Bool)
in' a lp = unsafeProjectSql' . SQL.paren
           $ SQL.in' (unsafeShowSql' a) (Projection.unsafeStringSqlList unsafeShowSql' lp)

-- | Operator corresponding SQL /IS NULL/ , and extended against record types.
isNothing :: (OperatorProjectable (Projection c), ProjectableShowSql (Projection c), HasColumnConstraint NotNull r)
          => Projection c (Maybe r) -> Projection c (Maybe Bool)
isNothing mr = unsafeProjectSql' $
               SQL.paren $ (SQL.defineBinOp SQL.IS)
               (Projection.unsafeStringSqlNotNullMaybe mr) SQL.NULL

-- | Operator corresponding SQL /NOT (... IS NULL)/ , and extended against record type.
isJust :: (OperatorProjectable (Projection c), ProjectableShowSql (Projection c), HasColumnConstraint NotNull r)
          => Projection c (Maybe r) -> Projection c (Maybe Bool)
isJust =  not' . isNothing

-- | Operator from maybe type using record extended 'isNull'.
fromMaybe :: (OperatorProjectable (Projection c), ProjectableShowSql (Projection c), HasColumnConstraint NotNull r)
          => Projection c r -> Projection c (Maybe r) -> Projection c r
fromMaybe d p = [ (isNothing p, d) ] `casesOrElse` unsafeCastProjectable p

unsafeUniTermFunction :: SqlProjectable p => Keyword -> p t
unsafeUniTermFunction =  unsafeProjectSql' . (SQL.<++> stringSQL "()")

-- | /RANK()/ term.
rank :: Integral a => Projection OverWindow a
rank =  unsafeUniTermFunction SQL.RANK

-- | /DENSE_RANK()/ term.
denseRank :: Integral a => Projection OverWindow a
denseRank =  unsafeUniTermFunction SQL.DENSE_RANK

-- | /ROW_NUMBER()/ term.
rowNumber :: Integral a => Projection OverWindow a
rowNumber =  unsafeUniTermFunction SQL.ROW_NUMBER

-- | /PERCENT_RANK()/ term.
percentRank :: Projection OverWindow Double
percentRank =  unsafeUniTermFunction SQL.PERCENT_RANK

-- | /CUME_DIST()/ term.
cumeDist :: Projection OverWindow Double
cumeDist =  unsafeUniTermFunction SQL.CUME_DIST

-- | Placeholder parameter type which has real parameter type arguemnt 'p'.
data PlaceHolders p = PlaceHolders

-- | Unsafely add placeholder parameter to queries.
unsafeAddPlaceHolders :: Functor f => f a -> f (PlaceHolders p, a)
unsafeAddPlaceHolders =  fmap ((,) PlaceHolders)

-- | Unsafely get placeholder parameter
unsafePlaceHolders :: PlaceHolders p
unsafePlaceHolders =  PlaceHolders

-- | No placeholder semantics
unitPlaceHolder :: PlaceHolders ()
unitPlaceHolder =  unsafePlaceHolders

-- | Unsafely cast placeholder parameter type.
unsafeCastPlaceHolders :: PlaceHolders a -> PlaceHolders b
unsafeCastPlaceHolders PlaceHolders = PlaceHolders

unsafeProjectPlaceHolder' :: (PersistableWidth r, SqlProjectable p)
                               => (PersistableRecordWidth r, p r)
unsafeProjectPlaceHolder' =  unsafeProjectSqlTerms . (`replicate` "?") <$> derivedWidth

unsafeProjectPlaceHolder :: (PersistableWidth r, SqlProjectable p)
                               => p r
unsafeProjectPlaceHolder =  snd unsafeProjectPlaceHolder'

-- | Provide scoped placeholder and return its parameter object.
placeholder' :: (PersistableWidth t, SqlProjectable p) => (p t -> a) ->  (PlaceHolders t, a)
placeholder' f = (PlaceHolders, f unsafeProjectPlaceHolder)

-- | Provide scoped placeholder and return its parameter object. Monadic version.
placeholder :: (PersistableWidth t, SqlProjectable p, Monad m) => (p t -> m a) -> m (PlaceHolders t, a)
placeholder f = do
  let (ph, ma) = placeholder' f
  a <- ma
  return (ph, a)


-- | Zipping projections.
projectZip :: ProjectableApplicative p => p a -> p b -> p (a, b)
projectZip pa pb = (,) |$| pa |*| pb

-- | Binary operator the same as 'projectZip'.
(><) :: ProjectableApplicative p => p a -> p b -> p (a, b)
(><) =  projectZip

-- | Interface to control 'Maybe' of phantom type in projections.
class ProjectableMaybe p where
  -- | Cast projection phantom type into 'Maybe'.
  just :: p a -> p (Maybe a)
  -- | Compose nested 'Maybe' phantom type on projection.
  flattenMaybe :: p (Maybe (Maybe a)) -> p (Maybe a)

-- | Control phantom 'Maybe' type in placeholder parameters.
instance ProjectableMaybe PlaceHolders where
  just         = unsafeCastPlaceHolders
  flattenMaybe = unsafeCastPlaceHolders

-- | Control phantom 'Maybe' type in projection type 'Projection'.
instance ProjectableMaybe (Projection c) where
  just         = Projection.just
  flattenMaybe = Projection.flattenMaybe

-- | Zipping except for identity element laws.
class ProjectableApplicative p => ProjectableIdZip p where
  leftId  :: p ((), a) -> p a
  rightId :: p (a, ()) -> p a

-- | Zipping except for identity element laws against placeholder parameter type.
instance ProjectableIdZip PlaceHolders where
  leftId  = unsafeCastPlaceHolders
  rightId = unsafeCastPlaceHolders

-- | Compose seed of record type 'PlaceHolders'.
instance ProjectableFunctor PlaceHolders where
  _ |$| PlaceHolders = PlaceHolders

-- | Compose record type 'PlaceHolders' using applicative style.
instance ProjectableApplicative PlaceHolders where
  pf |*| pa = unsafeCastPlaceHolders (pf >< pa)

infixl 7 .*., ./., ?*?, ?/?
infixl 6 .+., .-., ?+?, ?-?
infixl 5 .||., ?||?
infix  4 .=., .<>., .>., .>=., .<., .<=., `in'`, `like`, `likeMaybe`, `like'`, `likeMaybe'`
infixr 3 `and'`
infixr 2 `or'`
infixl 1  ><