add nodeLabMaybe and edgeLabMaybe

src/IGraph.hs

@@ -69,11 +69,25 @@ class MGraph d => Graph d where
     nodeLab (LGraph g _) i = read $ igraphCattributeVAS g vertexAttr i
     {-# INLINE nodeLab #-}
 
+    nodeLabMaybe :: Read v => LGraph d v e -> Node -> Maybe v
+    nodeLabMaybe gr@(LGraph g _) i =
+        if igraphCattributeHasAttr g IgraphAttributeVertex vertexAttr
+            then Just $ nodeLab gr i
+            else Nothing
+    {-# INLINE nodeLabMaybe #-}
+
     edgeLab :: Read e => LGraph d v e -> Edge -> e
     edgeLab (LGraph g _) (fr,to) = read $ igraphCattributeEAS g edgeAttr $
                                    igraphGetEid g fr to True True
     {-# INLINE edgeLab #-}
 
+    edgeLabMaybe :: Read e => LGraph d v e -> Edge -> Maybe e
+    edgeLabMaybe gr@(LGraph g _) i =
+        if igraphCattributeHasAttr g IgraphAttributeEdge edgeAttr
+            then Just $ edgeLab gr i
+            else Nothing
+    {-# INLINE edgeLabMaybe #-}
+
     edgeLabByEid :: Read e => LGraph d v e -> Int -> e
     edgeLabByEid (LGraph g _) i = read $ igraphCattributeEAS g edgeAttr i
     {-# INLINE edgeLabByEid #-}

src/IGraph/Internal/Attribute.chs

@@ -11,6 +11,7 @@ import System.IO.Unsafe (unsafePerformIO)
 
 {#import IGraph.Internal.Graph #}
 {#import IGraph.Internal.Data #}
+{#import IGraph.Internal.Constants #}
 
 #include "igraph/igraph.h"
 
@@ -37,7 +38,7 @@ instance Storable AttributeRecord where
         withStrVectorPtr vptr $ \ptr ->
             {#set igraph_attribute_record_t.value #} p $ castPtr ptr
 
-{#fun pure igraph_cattribute_has_attr as ^ { `IGraphPtr', `Int', `String' } -> `Bool' #}
+{#fun pure igraph_cattribute_has_attr as ^ { `IGraphPtr', `AttributeElemtype', `String' } -> `Bool' #}
 
 {#fun igraph_cattribute_GAN_set as ^ { `IGraphPtr', `String', `Double' } -> `Int' #}
 

src/IGraph/Internal/Constants.chs

@@ -12,3 +12,5 @@ import Foreign
 {#enum igraph_spincomm_update_t as SpincommUpdate {underscoreToCase} deriving (Show, Eq) #}
 
 {#enum igraph_spinglass_implementation_t as SpinglassImplementation {underscoreToCase} deriving (Show, Eq) #}
+
+{#enum igraph_attribute_elemtype_t as AttributeElemtype {underscoreToCase} deriving (Show, Eq) #}