Some notes on parsers

Evaluator.cs

@@ -5,4 +5,6 @@ public class Evaluator {
       String: s => s.ToString()
     );
   }
-}
+}
+
+// Note: for typeclass resolution, ask that functions have their parameters and return types annotated. This annotation is added to the values at run-time, which allows to dispatch based on the annotation rather than on the actual value.

Parser.cs

@@ -2,23 +2,39 @@ using System;
 using System.Text;
 using System.Collections.Generic;
 using System.Linq;
-using System.Globalization;
+using Immutable;
 using S = Lexer.S;
 using static Global;
 
 public static class Parser {
   public static Ast.Expr Parse(string source) {
-    foreach (var lexeme in Lexer.Lex(source)) {
-      return lexeme.state.Match(
-        Int: () => Ast.Expr.Int(Int32.Parse(lexeme.lexeme)),
-        String: () => Ast.Expr.String(lexeme.lexeme),
-        Space: () => throw new NotImplementedException(), // ignore
-        End: () => throw new NotImplementedException(),
-        Decimal: () => throw new NotImplementedException(),
-        StringOpen: () => throw new NotImplementedException(),
-        StringClose: () => throw new NotImplementedException()
-      );
-    }
-    throw new Exception("empty file, rm this when consuming the whole stream of lexemes.");
+    return Lexer.Lex(source)
+      .SelectMany(lexeme =>
+        lexeme.state.Match(
+          Int: () => Ast.Expr.Int(Int32.Parse(lexeme.lexeme)).Singleton(),
+          String: () => Ast.Expr.String(lexeme.lexeme).Singleton(),
+          Space: () => Enumerable.Empty<Ast.Expr>(), // ignore
+          End: () => Enumerable.Empty<Ast.Expr>(),
+          Decimal: () => Enumerable.Empty<Ast.Expr>(),
+          StringOpen: () => Enumerable.Empty<Ast.Expr>(),
+          StringClose: () => Enumerable.Empty<Ast.Expr>()
+        )
+      )
+      .Single()
+      .ElseThrow(() => new Exception("empty file or more than one expression in file."));
   }
-}
+}
+
+// Notes:
+
+// (a, b, c) is parsed as (expr (paren (expr comma (expr a) (expr comma (expr b) (expr c))))) where expr is a run-time wrapper allowing e.g. passing an explicit environment or (useful in this case) distinguish between a tuple-value referenced by c and a paren expression. In contrast, (a, (b, c)) is parsed as (expr (paren (expr comma (expr a) (expr paren (expr comma (expr b) (expr c))))))
+
+// (a < b <= c < d > e) is parsed similarly as the sequence of commas, allowing the comparison operators to compare their predecessor instead of the boolean output value.
+
+// ("if" condition "then" clause) returns a boolean-like value, indicating what the original condition was. It's as simple as (operator ("if" condition "then" clause) = real_if condition real_then { clause with condition_was = true } real_else { condition_was = false }). (ifthen "else" clause) is just a binary operator.
+
+// It is also possible to have the "else" operator taks an AST as its left operand, and inspect it to extract and rewrite the "if".
+
+// -3 is recognized by the lexer, but -x is not allowed. Otherwise f -x is ambiguous, could be f (-x) or (f) - (x)
+
+// relaxed unicity: the symbols must not appear in other operators of the same namespace nor as the closing bracket symbols which delimit the uses of this namespace in closed operators. Rationale: once the closing bracket is known, if the entire sub-expression doesn't include that bracket then the parser can fast-forward until the closing bracket, only caring about matching open and close symbols which may delimit sub-expressions with different namespaces, and know that whatever's inside is unambiguous.

Utils/Enumerable.cs

@@ -130,7 +130,6 @@ public static class Collection {
 
   public static Option<T> First<T>(this IEnumerable<T> ie, Func<T, bool> predicate) {
     var e = ie.GetEnumerator();
-    bool found = false;
     while (e.MoveNext()) {
       if (predicate(e.Current)) {
         return e.Current.Some();
@@ -180,7 +179,7 @@ public static class Collection {
       //this.dictionary = dictionary;
     }
     
-    public TValue this[TKey key] {
+    public new TValue this[TKey key] {
       get {
         return this.GetOrDefault(key, defaultValue);
       }