implement more shorthand functions

Nix/Expr/Shorthands.hs

@@ -100,10 +100,24 @@ mkFunction params = Fix . NAbs params
 mkDot :: NExpr -> Text -> NExpr
 mkDot e key = Fix $ NSelect e [StaticKey key] Nothing
 
+-- | An `inherit` clause without an expression to pull from.
+inherit :: [NKeyName e] -> Binding e
+inherit = Inherit Nothing
+
+-- | An `inherit` clause with an expression to pull from.
+inheritFrom :: e -> [NKeyName e] -> Binding e
+inheritFrom expr = Inherit (Just expr)
+
 -- | Shorthand for producing a binding of a name to an expression.
 bindTo :: Text -> NExpr -> Binding NExpr
 bindTo name val = NamedVar (mkSelector name) val
 
+-- | Infix version of bindTo.
+($=) :: Text -> NExpr -> Binding NExpr
+name $= value = bindTo name value
+
+infixr 2 $=
+
 -- | Append a list of bindings to a set or let expression.
 -- For example, adding `[a = 1, b = 2]` to `let c = 3; in 4` produces
 -- `let a = 1; b = 2; c = 3; in 4`.

Nix/Expr/Types.hs

@@ -71,6 +71,11 @@ data NExprF r
   -- ^ Assert that the first returns true before evaluating the second.
   deriving (Ord, Eq, Generic, Typeable, Data, Functor, Show)
 
+-- | We make an `IsString` for expressions, where the string is interpreted
+-- as an identifier. This is the most common use-case...
+instance IsString NExpr where
+  fromString = Fix . NSym . fromString
+
 -- | The monomorphic expression type is a fixed point of the polymorphic one.
 type NExpr = Fix NExprF
 
@@ -153,6 +158,10 @@ data NKeyName r
   | StaticKey !Text
   deriving (Eq, Ord, Generic, Typeable, Data, Show)
 
+-- | Most key names are just static text, so this instance is convenient.
+instance IsString (NKeyName r) where
+  fromString = StaticKey . fromString
+
 -- | Deriving this instance automatically is not possible because @r@
 -- occurs not only as last argument in @Antiquoted (NString r) r@
 instance Functor NKeyName where