[poincare] Add UntypedBuilder Function and Symbol

UntypedBuilders moreover retrieve Expression value from context.

poincare/include/poincare/function.h

@@ -51,6 +51,15 @@ public:
   Function(const char * name, size_t length, Expression child) : Function(name, length) {
     replaceChildAtIndexInPlace(0, child);
   }
+  static Expression UntypedBuilder(const char * name, size_t length, Expression child, Context * context) {
+    // create an expression only if it is not in the context or defined as a function
+    Function f(name, length, child);
+    if (SymbolAbstract::ValidInContext(f, context)) {
+      return f;
+    }
+    return Expression();
+  }
+
 
   Expression replaceSymbolWithExpression(const SymbolAbstract & symbol, const Expression & expression);
   Expression shallowReduce(Context & context, Preferences::AngleUnit angleUnit, bool replaceSymbols = true);

poincare/include/poincare/symbol.h

@@ -57,6 +57,14 @@ public:
   Symbol(char name);
   Symbol(const SymbolNode * node) : SymbolAbstract(node) {}
 
+  static Expression UntypedBuilder(const char * name, size_t length, Context * context) {
+    // create an expression only if it is not in the context or defined as a symbol
+    Symbol s(name, length);
+    if (SymbolAbstract::ValidInContext(s, context)) {
+      return s;
+    }
+    return Expression();
+  }
   // Symbol properties
   static bool isSeriesSymbol(const char * c);
   static bool isRegressionSymbol(const char * c);

poincare/include/poincare/symbol_abstract.h

@@ -56,6 +56,11 @@ class SymbolAbstract : public Expression {
 public:
   const char * name() const { return node()->name(); }
   static size_t TruncateExtension(char * dst, const char * src, size_t len);
+  static bool ValidInContext(SymbolAbstract & s, Context * context) {
+    // Retrive from context the expression corresponding to s
+    Expression f = context ? context->expressionForSymbol(s) : Expression();
+    return f.isUninitialized() || f.type() == s.type();
+  }
   constexpr static size_t k_maxNameSize = 8;
 protected:
   SymbolAbstract(const SymbolAbstractNode * node) : Expression(node) {}