40 lines
2.5 KiB
C#
40 lines
2.5 KiB
C#
using System;
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using System.Text;
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using System.Collections.Generic;
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using System.Linq;
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using Immutable;
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using S = Lexer.S;
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using static Global;
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public static class Parser {
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public static Ast.Expr Parse(string source) {
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return Lexer.Lex(source)
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.SelectMany(lexeme =>
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lexeme.state.Match(
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Int: () => Ast.Expr.Int(Int32.Parse(lexeme.lexeme)).Singleton(),
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String: () => Ast.Expr.String(lexeme.lexeme).Singleton(),
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Space: () => Enumerable.Empty<Ast.Expr>(), // ignore
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End: () => Enumerable.Empty<Ast.Expr>(),
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Decimal: () => Enumerable.Empty<Ast.Expr>(),
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StringOpen: () => Enumerable.Empty<Ast.Expr>(),
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StringClose: () => Enumerable.Empty<Ast.Expr>()
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)
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)
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.Single()
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.ElseThrow(() => new Exception("empty file or more than one expression in file."));
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}
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}
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// Notes:
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// (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))))))
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// (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.
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// ("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.
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// 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".
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// -3 is recognized by the lexer, but -x is not allowed. Otherwise f -x is ambiguous, could be f (-x) or (f) - (x)
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// 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. |