Add fromMaybeTable (#179)

A little helper function for `MaybeTable`s analogous to `Data.Maybe.fromMaybe`.

src/Rel8.hs

@@ -58,6 +58,7 @@ module Rel8
   , MaybeTable
   , maybeTable, ($?), nothingTable, justTable
   , isNothingTable, isJustTable
+  , fromMaybeTable
   , optional
   , catMaybeTable
   , traverseMaybeTable
@@ -114,7 +115,7 @@ module Rel8
   , isNullTable, isNonNullTable
   , catNullTable
   , nameNullTable
-  , fromMaybeTable, toMaybeTable
+  , toNullTable, toMaybeTable
 
     -- ** Algebraic data types / sum types
     -- $adts

src/Rel8/Table/Maybe.hs

@@ -15,6 +15,7 @@ module Rel8.Table.Maybe
   ( MaybeTable(..)
   , maybeTable, nothingTable, justTable
   , isNothingTable, isJustTable
+  , fromMaybeTable
   , ($?)
   , aggregateMaybeTable
   , nameMaybeTable
@@ -206,6 +207,11 @@ justTable :: a -> MaybeTable Expr a
 justTable = MaybeTable mempty . pure
 
 
+-- | 'Data.Maybe.fromMaybe' for 'MaybeTable's.
+fromMaybeTable :: Table Expr a => a -> MaybeTable Expr a -> a
+fromMaybeTable fallback = maybeTable fallback id
+
+
 -- | Project a single expression out of a 'MaybeTable'. You can think of this
 -- operator like the '$' operator, but it also has the ability to return
 -- @null@.

src/Rel8/Table/Null.hs

@@ -13,7 +13,7 @@ module Rel8.Table.Null
   , nullableTable, nullTable, nullifyTable, unsafeUnnullifyTable
   , isNullTable, isNonNullTable
   , nameNullTable
-  , toMaybeTable, fromMaybeTable
+  , toMaybeTable, toNullTable
   )
 where
 
@@ -131,5 +131,5 @@ toMaybeTable = nullableTable nothingTable justTable
 
 -- | Convert a 'MaybeTable' to a 'NullTable'. Note that if the underlying @a@
 -- has no non-nullable fields, this is a lossy conversion.
-fromMaybeTable :: Table Expr a => MaybeTable Expr a -> NullTable Expr a
-fromMaybeTable = maybeTable nullTable nullifyTable
+toNullTable :: Table Expr a => MaybeTable Expr a -> NullTable Expr a
+toNullTable = maybeTable nullTable nullifyTable