Database.HaskRel.Support

Predicates and aggregates useful to relational querying. These are not part of the relational algebra but are useful in relational queries in restriction predicates, and extension and summarization expressions. It also has great utility in non-relational queries, such as when performing aggregations.

Predicates

(&=) :: a -> (a -> Bool) -> Bool infix 5

Reverse predicate application. As "Data.Function.&", the reverse function application operator, but restricted to a boolean result.

plusminus :: (Ord a, Num a) => a -> a -> a -> Bool

>>> plusminus 9 3 10
True

(+-) :: (Ord a, Num a) => a -> a -> a -> Bool infix 7

>>> 10 &= 9 +- 3
True

(±) :: (Ord a, Num a) => a -> a -> a -> Bool infix 7

>>> 10 &= 9 ± 3
True
>>> rPrint$ p ∣ (\[pun|weight|] -> weight &= 9 ± 3)
┌─────┬───────┬───────┬────────┬────────┐
│ pno │ pName │ color │ weight │ city   │
╞═════╪═══════╪═══════╪════════╪════════╡
│ P1  │ Nut   │ Red   │ 12 % 1 │ London │
│ P5  │ Cam   │ Blue  │ 12 % 1 │ Paris  │
└─────┴───────┴───────┴────────┴────────┘

between :: Ord a => a -> (a, a) -> Bool

>>> 5 `between` (5,9)
True

(>=<=) :: Ord a => a -> a -> a -> Bool

>>> 5 &= 5 >=<= 9
True

(≥≤) :: Ord a => a -> a -> a -> Bool

>>> 5 &= 5 ≥≤ 9
True
>>> rPrint$ p ∣ (\[pun|weight|] -> weight &= 11 ≥≤ 14)
┌─────┬───────┬───────┬────────┬────────┐
│ pno │ pName │ color │ weight │ city   │
╞═════╪═══════╪═══════╪════════╪════════╡
│ P1  │ Nut   │ Red   │ 12 % 1 │ London │
│ P4  │ Screw │ Red   │ 14 % 1 │ London │
│ P5  │ Cam   │ Blue  │ 12 % 1 │ Paris  │
└─────┴───────┴───────┴────────┴────────┘

xBetween :: Ord a => a -> (a, a) -> Bool

Between exclusive

>>> 5 `xBetween` (5,9)
False

(><) :: Ord a => a -> a -> a -> Bool

>>> 5 &= 5 >< 9
False
>>> rPrint$ p ∣ (\[pun|weight|] -> weight &= 11 >< 14)
┌─────┬───────┬───────┬────────┬────────┐
│ pno │ pName │ color │ weight │ city   │
╞═════╪═══════╪═══════╪════════╪════════╡
│ P1  │ Nut   │ Red   │ 12 % 1 │ London │
│ P5  │ Cam   │ Blue  │ 12 % 1 │ Paris  │
└─────┴───────┴───────┴────────┴────────┘

(>=<) :: Ord a => a -> a -> a -> Bool

>>> 5 &= 5 >=< 9
True

(≥<) :: Ord a => a -> a -> a -> Bool

>>> 5 &= 5 ≥< 9
True

(><=) :: Ord a => a -> a -> a -> Bool

>>> 5 &= 5 ><= 9
False

(>≤) :: Ord a => a -> a -> a -> Bool

>>> 5 &= 5 >≤ 9
False

(≤) :: Ord a => a -> a -> Bool

Synonym for <=

(≥) :: Ord a => a -> a -> Bool

Synonym for >=

(≠) :: Eq a => a -> a -> Bool

Synonym for /=

Aggregates

avg :: (Fractional a, Real a1) => [a1] -> a

Average of a list of values

>>> let _qtys a = Label .=. a :: Tagged "qtys" Double
>>> pt$ group sp (rHdr (pno,qty)) (_qtys . avg . agg qty)
┌────────────────────┬───────────────┐
│ qtys :: Double     │ sno :: String │
╞════════════════════╪═══════════════╡
│ 200.0              │ S3            │
│ 216.66666666666666 │ S1            │
│ 300.0              │ S4            │
│ 350.0              │ S2            │
└────────────────────┴───────────────┘

Note the explicit type in the definition of _qtys; it is neccessary to provide a specific type for pt to format it as a table, even when an expression would otherwise be correct without this. The following would blow up if prefixed with "pt$":

>>> group sp (rHdr (pno,qty)) ((qtys .=.) . avg . agg qty)
fromList [Record{qtys=200.0,sno="S3"},Record{qtys=216.66666666666666,sno="S1"},Record{qtys=300.0,sno="S4"},Record{qtys=350.0,sno="S2"}]

minx :: Ord a => [a] -> a -> a

Minimum of several values, defaulting to the second argument if there are no elements.

>>> pt$ group sp (rHdr (pno,qty)) ((qtys .=.) . (`minx` 0) . agg qty)
┌─────────────────┬───────────────┐
│ qtys :: Integer │ sno :: String │
╞═════════════════╪═══════════════╡
│ 100             │ S1            │
│ 200             │ S3            │
│ 200             │ S4            │
│ 300             │ S2            │
└─────────────────┴───────────────┘

maxx :: Ord a => [a] -> a -> a

Maximum of several values, defaulting to the second argument if there are no elements

>>> pt$ group sp (rHdr (pno,qty)) ((qtys .=.) . (`maxx` 0) . agg qty)
┌─────────────────┬───────────────┐
│ qtys :: Integer │ sno :: String │
╞═════════════════╪═══════════════╡
│ 200             │ S3            │
│ 400             │ S1            │
│ 400             │ S2            │
│ 400             │ S4            │
└─────────────────┴───────────────┘