Refactor schema expressions

extensions/wikidata/src/org/openrefine/wikidata/schema/WbDateConstant.java

@@ -17,7 +17,7 @@ import com.fasterxml.jackson.annotation.JsonProperty;
 import com.google.common.collect.ImmutableMap;
 
 
-public class WbDateConstant extends WbDateExpr {
+public class WbDateConstant implements WbValueExpr<TimeValue> {
 
     public static Map<SimpleDateFormat,Integer> acceptedFormats = ImmutableMap.<SimpleDateFormat,Integer>builder()
         .put(new SimpleDateFormat("yyyy"), 9)

extensions/wikidata/src/org/openrefine/wikidata/schema/WbDateExpr.java

@@ -1,11 +0,0 @@
-package org.openrefine.wikidata.schema;
-
-import org.openrefine.wikidata.schema.exceptions.SkipSchemaExpressionException;
-import org.wikidata.wdtk.datamodel.interfaces.TimeValue;
-
-public abstract class WbDateExpr extends WbValueExpr {
-
-    @Override
-    public abstract TimeValue evaluate(ExpressionContext ctxt)
-            throws SkipSchemaExpressionException;
-}

extensions/wikidata/src/org/openrefine/wikidata/schema/WbDateVariable.java

@@ -5,38 +5,19 @@ import java.text.ParseException;
 import org.openrefine.wikidata.schema.exceptions.SkipSchemaExpressionException;
 import org.wikidata.wdtk.datamodel.interfaces.TimeValue;
 
-import com.fasterxml.jackson.annotation.JsonCreator;
-import com.fasterxml.jackson.annotation.JsonProperty;
-
 import com.google.refine.model.Cell;
 
 
-public class WbDateVariable extends WbDateExpr {
+public class WbDateVariable extends WbVariableExpr<TimeValue> {
-    
-    private String columnName;
-    
-    @JsonCreator
-    public WbDateVariable(
-            @JsonProperty("columnName") String columnName) {
-        this.columnName = columnName;
-    }
 
     @Override
-    public TimeValue evaluate(ExpressionContext ctxt)
+    public TimeValue fromCell(Cell cell, ExpressionContext ctxt)
             throws SkipSchemaExpressionException {
-        Cell cell = ctxt.getCellByName(columnName);
+        try {
-        if (cell != null) {
+            // TODO accept parsed dates (without converting them to strings)
-            try {
+            return WbDateConstant.parse(cell.value.toString());
-                // TODO accept parsed dates (without converting them to strings)
+        } catch (ParseException e) {
-                return WbDateConstant.parse(cell.value.toString());
+            throw new SkipSchemaExpressionException();
-            } catch (ParseException e) {
-            }
         }
-        throw new SkipSchemaExpressionException();
-    }
-
-    @JsonProperty("columnName")
-    public String getColumnName() {
-        return columnName;
     }
 }

extensions/wikidata/src/org/openrefine/wikidata/schema/WbItemConstant.java

@@ -1,17 +1,16 @@
 package org.openrefine.wikidata.schema;
 
-import org.openrefine.wikidata.schema.entityvalues.ReconItemIdValue;
 import org.openrefine.wikidata.schema.entityvalues.SuggestedItemIdValue;
-import org.wikidata.wdtk.datamodel.implementation.ItemIdValueImpl;
 import org.wikidata.wdtk.datamodel.interfaces.ItemIdValue;
 
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
 
-
+/**
-public class WbItemConstant extends WbItemExpr {
+ * Represents an item that does not vary,
-    /* Represents an item that does not vary,
+ * it is independent of the row.
-     * it is independent of the row. */
+ */
+public class WbItemConstant implements WbValueExpr<ItemIdValue> {
     
     private String qid;
     private String label;

extensions/wikidata/src/org/openrefine/wikidata/schema/WbItemDocumentExpr.java

@@ -10,16 +10,22 @@ import org.wikidata.wdtk.datamodel.interfaces.Statement;
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
 
-
+/**
+ * The representation of an item document, which can contain
+ * variables both for its own id and in its contents.
+ * 
+ * @author antonin
+ *
+ */
 public class WbItemDocumentExpr extends JacksonJsonizable {
 
-    private WbItemExpr subject;
+    private WbValueExpr<? extends ItemIdValue> subject;
     private List<WbNameDescExpr> nameDescs;
     private List<WbStatementGroupExpr> statementGroups;
     
     @JsonCreator
     public WbItemDocumentExpr(
-            @JsonProperty("subject") WbItemExpr subjectExpr,
+            @JsonProperty("subject") WbValueExpr<? extends ItemIdValue> subjectExpr,
             @JsonProperty("nameDescs") List<WbNameDescExpr> nameDescExprs,
             @JsonProperty("statementGroups") List<WbStatementGroupExpr> statementGroupExprs) {
         this.subject = subjectExpr;
@@ -46,7 +52,7 @@ public class WbItemDocumentExpr extends JacksonJsonizable {
     }
 
     @JsonProperty("subject")
-    public WbItemExpr getSubject() {
+    public WbValueExpr<? extends ItemIdValue> getSubject() {
         return subject;
     }
 

extensions/wikidata/src/org/openrefine/wikidata/schema/WbItemExpr.java

@@ -1,11 +0,0 @@
-package org.openrefine.wikidata.schema;
-
-import org.openrefine.wikidata.schema.ExpressionContext;
-import org.openrefine.wikidata.schema.WbValueExpr;
-import org.openrefine.wikidata.schema.exceptions.SkipSchemaExpressionException;
-import org.wikidata.wdtk.datamodel.interfaces.ItemIdValue;
-
-
-public abstract class WbItemExpr extends WbValueExpr {
-    public abstract ItemIdValue evaluate(ExpressionContext ctxt) throws SkipSchemaExpressionException;
-}

extensions/wikidata/src/org/openrefine/wikidata/schema/WbItemVariable.java

@@ -5,36 +5,24 @@ import org.openrefine.wikidata.schema.entityvalues.ReconItemIdValue;
 import org.openrefine.wikidata.schema.exceptions.SkipSchemaExpressionException;
 import org.wikidata.wdtk.datamodel.interfaces.ItemIdValue;
 
-import com.fasterxml.jackson.annotation.JsonCreator;
-import com.fasterxml.jackson.annotation.JsonProperty;
-
 import com.google.refine.model.Cell;
 import com.google.refine.model.Recon.Judgment;
 
-public class WbItemVariable extends WbItemExpr {
+/**
-    /* An item that depends on a reconciled value in a column */
+ * An item that depends on a reconciled value in a column.
-    
+ * 
-    private String columnName;
+ * @author antonin
-    
+ *
-    @JsonCreator
+ */
-    public WbItemVariable(
+public class WbItemVariable extends WbVariableExpr<ItemIdValue> {
-            @JsonProperty("columnName") String columnName) {
-        this.columnName = columnName;
-    }
 
     @Override
-    public ItemIdValue evaluate(ExpressionContext ctxt) throws SkipSchemaExpressionException {
+    public ItemIdValue fromCell(Cell cell, ExpressionContext ctxt) throws SkipSchemaExpressionException {
-        Cell cell = ctxt.getCellByName(getColumnName());
+        if (cell.recon != null
-        if (cell != null && cell.recon != null
                 && (Judgment.Matched.equals(cell.recon.judgment) ||
                     Judgment.New.equals(cell.recon.judgment))) {
             return new ReconItemIdValue(cell.recon, cell.value.toString());
         }
         throw new SkipSchemaExpressionException();
     }
-
-    @JsonProperty("columnName")
-    public String getColumnName() {
-        return columnName;
-    }
 }

extensions/wikidata/src/org/openrefine/wikidata/schema/WbLanguageConstant.java

@@ -5,7 +5,15 @@ import org.openrefine.wikidata.schema.exceptions.SkipSchemaExpressionException;
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
 
-public class WbLanguageConstant extends WbLanguageExpr {
+/**
+ * A constant that represents a Wikimedia language code.
+ * 
+ * TODO: migrate to a class more specific than String, with validation.
+ * 
+ * @author antonin
+ *
+ */
+public class WbLanguageConstant implements WbValueExpr<String> {
     
     protected String _langId;
     protected String _langLabel;

extensions/wikidata/src/org/openrefine/wikidata/schema/WbLanguageExpr.java

@@ -1,26 +0,0 @@
-package org.openrefine.wikidata.schema;
-
-import org.openrefine.wikidata.schema.exceptions.SkipSchemaExpressionException;
-import org.openrefine.wikidata.utils.JacksonJsonizable;
-
-import com.fasterxml.jackson.annotation.JsonSubTypes;
-import com.fasterxml.jackson.annotation.JsonTypeInfo;
-import com.fasterxml.jackson.annotation.JsonSubTypes.Type;
-
-@JsonTypeInfo(use=JsonTypeInfo.Id.NAME,
-              include=JsonTypeInfo.As.PROPERTY,
-              property="type")
-@JsonSubTypes({ 
-    @Type(value = WbLanguageConstant.class, name = "wblanguageconstant"), 
-    @Type(value = WbLanguageVariable.class, name = "wblanguagevariable")
-})
-public abstract class WbLanguageExpr extends JacksonJsonizable {
-    /**
-     * Evaluates the language expression to a Wikimedia language code
-     * 
-     * @param ctxt the evaluation context
-     * @return a Wikimedia language code
-     * @throws SkipSchemaExpressionException when the code is invalid
-     */
-    public abstract String evaluate(ExpressionContext ctxt) throws SkipSchemaExpressionException;
-}

extensions/wikidata/src/org/openrefine/wikidata/schema/WbLanguageVariable.java

@@ -7,31 +7,18 @@ import com.fasterxml.jackson.annotation.JsonProperty;
 
 import com.google.refine.model.Cell;
 
-
+/**
-public class WbLanguageVariable extends WbLanguageExpr {
+ * A language variable generates a language code from a cell.
-    
+ */
-    private String columnName;
+public class WbLanguageVariable extends WbVariableExpr<String> {
-    
-    @JsonCreator
-    public WbLanguageVariable(
-            @JsonProperty("columnName") String columnName) {
-        this.columnName = columnName;
-    }
 
     @Override
-    public String evaluate(ExpressionContext ctxt)
+    public String fromCell(Cell cell, ExpressionContext ctxt)
             throws SkipSchemaExpressionException {
-        Cell cell = ctxt.getCellByName(getColumnName());
+        if (cell.value != null && !cell.value.toString().isEmpty()) {
-        if (cell != null && cell.value != null && !cell.value.toString().isEmpty()) {
             // TODO some validation here?
             return cell.value.toString();
         }
         throw new SkipSchemaExpressionException();
     }
-
-    @JsonProperty("columnName")
-    public String getColumnName() {
-        return columnName;
-    }
-
 }

extensions/wikidata/src/org/openrefine/wikidata/schema/WbLocationConstant.java

@@ -10,7 +10,7 @@ import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
 
 
-public class WbLocationConstant extends WbLocationExpr {
+public class WbLocationConstant implements WbValueExpr<GlobeCoordinatesValue> {
     
     private String value;
     private GlobeCoordinatesValue parsed;

extensions/wikidata/src/org/openrefine/wikidata/schema/WbLocationExpr.java

@@ -1,10 +0,0 @@
-package org.openrefine.wikidata.schema;
-
-import org.openrefine.wikidata.schema.exceptions.SkipSchemaExpressionException;
-import org.wikidata.wdtk.datamodel.interfaces.GlobeCoordinatesValue;
-
-public abstract class WbLocationExpr extends WbValueExpr {
-    @Override
-    public abstract GlobeCoordinatesValue evaluate(ExpressionContext ctxt)
-            throws SkipSchemaExpressionException;
-}

extensions/wikidata/src/org/openrefine/wikidata/schema/WbLocationVariable.java

@@ -5,38 +5,19 @@ import java.text.ParseException;
 import org.openrefine.wikidata.schema.exceptions.SkipSchemaExpressionException;
 import org.wikidata.wdtk.datamodel.interfaces.GlobeCoordinatesValue;
 
-import com.fasterxml.jackson.annotation.JsonCreator;
-import com.fasterxml.jackson.annotation.JsonProperty;
-
 import com.google.refine.model.Cell;
 
 
-public class WbLocationVariable extends WbLocationExpr {
+public class WbLocationVariable extends WbVariableExpr<GlobeCoordinatesValue> {
-    
-    private String columnName;
-    
-    @JsonCreator
-    public WbLocationVariable(
-            @JsonProperty("columnName") String columnName) {
-        this.columnName = columnName;
-    }
 
     @Override
-    public GlobeCoordinatesValue evaluate(ExpressionContext ctxt)
+    public GlobeCoordinatesValue fromCell(Cell cell, ExpressionContext ctxt)
             throws SkipSchemaExpressionException {
-        Cell cell = ctxt.getCellByName(getColumnName());
+        String expr = cell.value.toString();
-        if (cell != null) {
+        try {
-            String expr = cell.value.toString();
+            return WbLocationConstant.parse(expr);
-            try {
+        } catch (ParseException e) {
-                return WbLocationConstant.parse(expr);
+            throw new SkipSchemaExpressionException();
-            } catch (ParseException e) {
-            }
         }
-        throw new SkipSchemaExpressionException();
-    }
-
-    @JsonProperty("columnName")
-    public String getColumnName() {
-        return columnName;
     }
 }

extensions/wikidata/src/org/openrefine/wikidata/schema/WbMonolingualExpr.java

@@ -4,20 +4,21 @@ import org.openrefine.wikidata.qa.QAWarning;
 import org.openrefine.wikidata.schema.exceptions.SkipSchemaExpressionException;
 import org.wikidata.wdtk.datamodel.helpers.Datamodel;
 import org.wikidata.wdtk.datamodel.interfaces.MonolingualTextValue;
+import org.wikidata.wdtk.datamodel.interfaces.StringValue;
 
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
 
 
-public class WbMonolingualExpr extends WbValueExpr {
+public class WbMonolingualExpr implements WbValueExpr<MonolingualTextValue> {
     
-    private WbLanguageExpr languageExpr;
+    private WbValueExpr<? extends String> languageExpr;
-    private WbStringExpr valueExpr;
+    private WbValueExpr<? extends StringValue> valueExpr;
     
     @JsonCreator
     public WbMonolingualExpr(
-            @JsonProperty("language") WbLanguageExpr languageExpr,
+            @JsonProperty("language") WbValueExpr<? extends String> languageExpr,
-            @JsonProperty("value") WbStringExpr valueExpr) {
+            @JsonProperty("value") WbValueExpr<? extends StringValue> valueExpr) {
         this.languageExpr = languageExpr;
         this.valueExpr = valueExpr;
     }
@@ -45,12 +46,12 @@ public class WbMonolingualExpr extends WbValueExpr {
     }
 
     @JsonProperty("language")
-    public WbLanguageExpr getLanguageExpr() {
+    public WbValueExpr<? extends String> getLanguageExpr() {
         return languageExpr;
     }
 
     @JsonProperty("value")
-    public WbStringExpr getValueExpr() {
+    public WbValueExpr<? extends StringValue> getValueExpr() {
         return valueExpr;
     }
 }

extensions/wikidata/src/org/openrefine/wikidata/schema/WbPropConstant.java

@@ -6,9 +6,13 @@ import org.wikidata.wdtk.datamodel.interfaces.PropertyIdValue;
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
 
-
+/**
-public class WbPropConstant extends WbPropExpr {
+ * A constant property, that does not change depending on the row
-    /* A constant property, that does not change depending on the row */
+ * 
+ * @author antonin
+ *
+ */
+public class WbPropConstant implements WbValueExpr<PropertyIdValue> {
     
     private String pid;
     private String label;

extensions/wikidata/src/org/openrefine/wikidata/schema/WbPropExpr.java

@@ -1,23 +0,0 @@
-package org.openrefine.wikidata.schema;
-
-import org.json.JSONException;
-import org.json.JSONObject;
-import org.wikidata.wdtk.datamodel.interfaces.PropertyIdValue;
-
-import com.fasterxml.jackson.annotation.JsonSubTypes;
-import com.fasterxml.jackson.annotation.JsonSubTypes.Type;
-
-
-@JsonSubTypes({ 
-    @Type(value = WbPropConstant.class, name = "wbpropconstant")
-})
-public abstract class WbPropExpr extends WbValueExpr {
-    /* An expression that represents a property */
-    
-    public abstract PropertyIdValue evaluate(ExpressionContext ctxt);
-    
-    public static WbPropExpr fromJSON(JSONObject obj) throws JSONException {
-        return WbPropConstant.fromJSON(obj);
-    }
-
-}

extensions/wikidata/src/org/openrefine/wikidata/schema/WbSnakExpr.java

@@ -13,13 +13,13 @@ import com.fasterxml.jackson.annotation.JsonProperty;
 
 public class WbSnakExpr extends JacksonJsonizable {
     
-    private WbPropExpr prop;
+    private WbValueExpr<? extends PropertyIdValue> prop;
-    private WbValueExpr value;
+    private WbValueExpr<? extends Value> value;
     
     @JsonCreator
     public WbSnakExpr(
-            @JsonProperty("prop") WbPropExpr propExpr,
+            @JsonProperty("prop") WbValueExpr<? extends PropertyIdValue> propExpr,
-            @JsonProperty("value") WbValueExpr valueExpr) {
+            @JsonProperty("value") WbValueExpr<? extends Value> valueExpr) {
         this.prop = propExpr;
         this.value = valueExpr;
     }
@@ -31,12 +31,12 @@ public class WbSnakExpr extends JacksonJsonizable {
     }
 
     @JsonProperty("prop")
-    public WbPropExpr getProp() {
+    public WbValueExpr<? extends PropertyIdValue> getProp() {
         return prop;
     }
 
     @JsonProperty("value")
-    public WbValueExpr getValue() {
+    public WbValueExpr<? extends Value> getValue() {
         return value;
     }
 }

extensions/wikidata/src/org/openrefine/wikidata/schema/WbStatementExpr.java

@@ -30,13 +30,13 @@ import com.fasterxml.jackson.annotation.JsonProperty;
 
 public class WbStatementExpr extends JacksonJsonizable {
     
-    private WbValueExpr mainSnakValueExpr;
+    private WbValueExpr<? extends Value> mainSnakValueExpr;
     private List<WbSnakExpr> qualifierExprs;
     private List<WbReferenceExpr> referenceExprs;
     
     @JsonCreator
     public WbStatementExpr(
-            @JsonProperty("value") WbValueExpr mainSnakValueExpr,
+            @JsonProperty("value") WbValueExpr<? extends Value> mainSnakValueExpr,
             @JsonProperty("qualifiers") List<WbSnakExpr> qualifierExprs,
             @JsonProperty("references") List<WbReferenceExpr> referenceExprs) {
         this.mainSnakValueExpr = mainSnakValueExpr;
@@ -99,7 +99,7 @@ public class WbStatementExpr extends JacksonJsonizable {
     }
 
     @JsonProperty("value")
-    public WbValueExpr getMainsnak() {
+    public WbValueExpr<? extends Value> getMainsnak() {
         return mainSnakValueExpr;
     }
 

extensions/wikidata/src/org/openrefine/wikidata/schema/WbStatementGroupExpr.java

@@ -17,12 +17,12 @@ import com.fasterxml.jackson.annotation.JsonProperty;
 
 public class WbStatementGroupExpr extends JacksonJsonizable {
     
-    private WbPropExpr propertyExpr;
+    private WbValueExpr<? extends PropertyIdValue> propertyExpr;
     private List<WbStatementExpr> statementExprs;
     
     @JsonCreator
     public WbStatementGroupExpr(
-            @JsonProperty("property") WbPropExpr propertyExpr,
+            @JsonProperty("property") WbValueExpr<? extends PropertyIdValue> propertyExpr,
             @JsonProperty("statements") List<WbStatementExpr> claimExprs) {
         this.propertyExpr = propertyExpr;
         this.statementExprs = claimExprs;
@@ -46,7 +46,7 @@ public class WbStatementGroupExpr extends JacksonJsonizable {
     }
 
     @JsonProperty("property")
-    public WbPropExpr getProperty() {
+    public WbValueExpr<? extends PropertyIdValue> getProperty() {
         return propertyExpr;
     }
 

extensions/wikidata/src/org/openrefine/wikidata/schema/WbStringConstant.java

@@ -7,7 +7,7 @@ import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
 
 
-public class WbStringConstant extends WbStringExpr {
+public class WbStringConstant implements WbValueExpr<StringValue> {
     
     private String value;
     

extensions/wikidata/src/org/openrefine/wikidata/schema/WbStringExpr.java

@@ -1,8 +0,0 @@
-package org.openrefine.wikidata.schema;
-
-import org.openrefine.wikidata.schema.exceptions.SkipSchemaExpressionException;
-import org.wikidata.wdtk.datamodel.interfaces.StringValue;
-
-public abstract class WbStringExpr extends WbValueExpr {
-    public abstract StringValue evaluate(ExpressionContext ctxt) throws SkipSchemaExpressionException;
-}

extensions/wikidata/src/org/openrefine/wikidata/schema/WbStringVariable.java

@@ -9,29 +9,20 @@ import com.fasterxml.jackson.annotation.JsonProperty;
 
 import com.google.refine.model.Cell;
 
-public class WbStringVariable extends WbStringExpr {
+/**
-    public static final String jsonType = "wbstringvariable";
+ * A variable that returns a simple string value.
-    
+ * 
-    private String columnName;
+ * @author antonin
-    
+ *
-    @JsonCreator
+ */
-    public WbStringVariable(
+public class WbStringVariable extends WbVariableExpr<StringValue> {
-            @JsonProperty("columnName") String columnName) {
-        this.columnName = columnName;
-    }
 
     @Override
-    public StringValue evaluate(ExpressionContext ctxt)
+    public StringValue fromCell(Cell cell, ExpressionContext ctxt)
             throws SkipSchemaExpressionException {
-        Cell cell = ctxt.getCellByName(columnName);
+        if (!cell.value.toString().isEmpty()) {
-        if (cell != null && !cell.value.toString().isEmpty()) {
             return Datamodel.makeStringValue(cell.value.toString());
         }
         throw new SkipSchemaExpressionException();
     }
-
-    @JsonProperty("columnName")
-    public String getColumnName() {
-        return columnName;
-    }
 }

extensions/wikidata/src/org/openrefine/wikidata/schema/WbValueExpr.java

@@ -10,7 +10,10 @@ import com.fasterxml.jackson.annotation.JsonSubTypes;
 import com.fasterxml.jackson.annotation.JsonTypeInfo;
 import com.fasterxml.jackson.annotation.JsonSubTypes.Type;
 
-
+/**
+ * The base interface for all expressions, which evaluate to a
+ * particular type T in an ExpressionContext.
+ */
 @JsonTypeInfo(use=JsonTypeInfo.Id.NAME,
       include=JsonTypeInfo.As.PROPERTY,
       property="type")
@@ -26,20 +29,15 @@ import com.fasterxml.jackson.annotation.JsonSubTypes.Type;
     @Type(value = WbDateConstant.class, name = "wbdateconstant"), 
     @Type(value = WbDateVariable.class, name = "wbdatevariable"),
     @Type(value = WbMonolingualExpr.class, name = "wbmonolingualexpr"),
+    @Type(value = WbPropConstant.class, name = "wbpropconstant"),
+    @Type(value = WbLanguageConstant.class, name = "wblanguageconstant"), 
+    @Type(value = WbLanguageVariable.class, name = "wblanguagevariable"),
   })
-public abstract class WbValueExpr extends JacksonJsonizable {
+public interface  WbValueExpr<T>  {
-    /* An expression that represents a Wikibase value,
-     * i.e. anything that can be on the right-hand side
-     * of a statement.
-     */
 
-    /*
+    /**
      * Evaluates the value expression in a given context,
-     * returns a wikibase value suitable to be the target of a claim.
+     * returns a Wikibase value suitable to be the target of a claim.
      */
-    public abstract Value evaluate(ExpressionContext ctxt) throws SkipSchemaExpressionException;
+    public T evaluate(ExpressionContext ctxt) throws SkipSchemaExpressionException;
-    
-    public static WbValueExpr fromJSON(JSONObject obj) throws JSONException {
-        return fromJSONClass(obj, WbValueExpr.class);
-    }
 }

extensions/wikidata/src/org/openrefine/wikidata/schema/WbVariableExpr.java

@@ -0,0 +1,91 @@
+package org.openrefine.wikidata.schema;
+
+import org.openrefine.wikidata.schema.exceptions.SkipSchemaExpressionException;
+import org.wikidata.wdtk.datamodel.interfaces.Value;
+
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+import com.google.refine.model.Cell;
+
+/**
+ * A base class for expressions which draw their values
+ * from a particular column.
+ * 
+ * @author antonin
+ *
+ * @param <T>
+ *      the type of Wikibase value returned by the expression.
+ */
+public abstract class WbVariableExpr<T> implements WbValueExpr<T> {
+
+    private String columnName;
+    
+    /**
+     * Constructs a variable expression from a column name.
+     * 
+     * @param columnName
+     *     the name of the column the expression should draw its value from.
+     */
+    @JsonCreator
+    public WbVariableExpr(
+            @JsonProperty("columnName") String columnName) {
+        this.columnName = columnName;
+    }
+    
+    /**
+     * Constructs a variable without setting the column name yet.
+     */
+    @JsonCreator
+    public WbVariableExpr() {
+        columnName = null;
+    }
+    
+    /**
+     * Returns the column name used by the variable.
+     * @return
+     *          the OpenRefine column name
+     */
+    @JsonProperty("columnName")
+    public String getColumnName() {
+        return columnName;
+    }
+    
+    /**
+     * Changes the column name used by the variable.
+     * This is useful for deserialization, as well as updates when
+     * column names change.
+     */
+    @JsonProperty("columnName")
+    public void setColumnName(String columnName) {
+        this.columnName = columnName;
+    }
+
+    /**
+     * Evaluates the expression in a given context, returning
+     */
+    @Override
+    public T evaluate(ExpressionContext ctxt)
+            throws SkipSchemaExpressionException {
+        Cell cell = ctxt.getCellByName(columnName);
+        if (cell != null) {
+            return fromCell(cell, ctxt);
+        }
+        throw new SkipSchemaExpressionException();
+    }
+    
+    /**
+     * Method that should be implemented by subclasses,
+     * converting an OpenRefine cell to a Wikibase value.
+     * Access to other values and emiting warnings is possible via
+     * the supplied EvaluationContext object.
+     * 
+     * @param cell
+     *          the cell to convert
+     * @param ctxt
+     *          the evaluation context
+     * @return
+     *          the corresponding Wikibase value
+     */
+    public abstract T fromCell(Cell cell, ExpressionContext ctxt) throws SkipSchemaExpressionException;
+}