Merge pull request #204 from corywalker/corywalker

Support Take[].

Author: corywalker

expreduce/builtin_list.go

@@ -614,6 +614,7 @@ func getListDefinitions() (defs []Definition) {
 			return applied
 		},
 	})
+	defs = append(defs, Definition{Name: "Take"})
 	defs = append(defs, Definition{Name: "Span"})
 	defs = append(defs, Definition{Name: "All"})
 	defs = append(defs, Definition{

expreduce/evalstate.go

@@ -556,12 +556,14 @@ func (es *EvalState) Define(lhs expreduceapi.Ex, rhs expreduceapi.Ex) {
 	newSpecificity := ruleSpecificity(heldLHS, rhs, name, es)
 	for i, dv := range es.defined.GetDef(name).Downvalues {
 		if dv.Specificity == 0 {
+			es.defined.LockKey(name)
 			dv.Specificity = ruleSpecificity(
 				dv.Rule.GetParts()[1],
 				dv.Rule.GetParts()[2],
 				name,
 				es,
 			)
+			es.defined.UnlockKey(name)
 		}
 		if dv.Specificity < newSpecificity {
 			newRule := atoms.NewExpression(

expreduce/resources.go

@@ -228,6 +228,25 @@
     ]
 };
 
+Take::usage = "`Take[list, m]` returns the first `m` items of `list`.";
+Take[list_, m_] := If[m > 0, list[[1;;m]], list[[m;;-1]]];
+Take[list_, {m_, n_}] := list[[m;;n]];
+Take[list_, {m_, n_, o_}] := list[[m;;n;;o]];
+Attributes[Take] = {NHoldRest, Protected};
+Tests`Take = {
+    ESimpleExamples[
+        EComment["Return the first two items in a list:"],
+        ESameTest[{1, 2}, Take[{1, 2, 3, 4}, 2]],
+        EComment["Return a range from the middle of a list:"],
+        ESameTest[{2, 3, 4}, Take[{1, 2, 3, 4, 5}, {2, 4}]]
+    ], EKnownFailures[
+        (* Return the last 2 items in a list: *)
+        ESameTest[{3, 4}, Take[{1, 2, 3, 4}, -2]],
+        (* Take every other item: *)
+        ESameTest[{1, 3, 5}, Take[{1, 2, 3, 4, 5}, {1, -1, 2}]]
+    ]
+};
+
 Span::usage = "`start ;; end` represents an index span to select using Part.";
 Attributes[Span] = {Protected};
 Tests`Span = {