Feature/#9 array functions (#57)

* #9 Added 'APPEND' function

* #9 Added 'FIRST' function

* #9 Added 'FLATTEN' function

* #9 Added 'INTERSECTION' function

* #9 Added 'LAST' function

* #9 Added 'MINUS' function

* #9 Added 'NTH' function

* #9 Added 'OUTERSECTION' function

* #9 Added 'POP' function

* #9 Added 'POSITION' function

* #9 Added 'PUSH' function

* Fixed nil pointer exception in value parser

* #9 Added 'REMOVE_NTH' function

* #9 Added 'REMOVE_VALUE' function

* #9 Added 'REMOVE_VALUES' function

* #9 Added 'REVERSE' function

* #9 Added 'SHIFT' function

* #9 Added 'SLICE' function

* Removed meme

* #9 Added 'SORTED' function

* #9 Added SORTED_UNIQUE function

* #9 Added 'UNION' function

* #9 Added 'UNION_DISTINCT' function

* #9 Added 'UNIQUE' function

* #9 Added 'UNSHIFT' function

* #9 Made more strict optional arg validation

* #9 Fixed linting errors

pkg/stdlib/arrays/unshift.go

@@ -0,0 +1,80 @@
+package arrays
+
+import (
+	"context"
+	"github.com/MontFerret/ferret/pkg/runtime/core"
+	"github.com/MontFerret/ferret/pkg/runtime/values"
+)
+
+/*
+ * Prepends value to a given array.
+ * @param array (Array) - Target array.
+ * @param value (Value) - Target value to prepend.
+ * @param unique (Boolean, optional) - Optional value indicating whether a value must be unique to be prepended.
+ * Default is false.
+ * @returns (Array) - New array with prepended value.
+ */
+func Unshift(_ context.Context, args ...core.Value) (core.Value, error) {
+	err := core.ValidateArgs(args, 2, 3)
+
+	if err != nil {
+		return values.None, err
+	}
+
+	err = core.ValidateType(args[0], core.ArrayType)
+
+	if err != nil {
+		return values.None, err
+	}
+
+	arr := args[0].(*values.Array)
+	value := args[1]
+	uniq := values.False
+
+	if len(args) > 2 {
+		err = core.ValidateType(args[2], core.BooleanType)
+
+		if err != nil {
+			return values.None, err
+		}
+
+		uniq = args[2].(values.Boolean)
+	}
+
+	result := values.NewArray(int(arr.Length() + 1))
+
+	if !uniq {
+		result.Push(value)
+
+		arr.ForEach(func(el core.Value, _ int) bool {
+			result.Push(el)
+
+			return true
+		})
+	} else {
+		ok := true
+
+		// let's just hope it's unique
+		// if not, we will terminate the loop and return a copy of an array
+		result.Push(value)
+
+		arr.ForEach(func(el core.Value, idx int) bool {
+			if el.Compare(value) != 0 {
+				result.Push(el)
+
+				return true
+			}
+
+			// not unique
+			ok = false
+			return false
+		})
+
+		if !ok {
+			// value is not unique, just return a new copy with same elements
+			return arr.Clone(), nil
+		}
+	}
+
+	return result, nil
+}