Adds IllegalState to ParserError (#24)

Also changes the `PairsExt::exactly_one` to use this error for the empty case.

Co-authored-by: Reza Handzalah <reza.handzalah@mbiz.co.id>

Author: tbmreza

partiql-parser/src/peg.rs

@@ -4,7 +4,7 @@
 //! can be exported for users to consume.
 
 use crate::prelude::*;
-use crate::result::syntax_error;
+use crate::result::{illegal_state, syntax_error};
 use pest::iterators::{Pair, Pairs};
 use pest::{Parser, RuleType};
 use pest_derive::Parser;
@@ -33,10 +33,7 @@ impl<'val, R: RuleType> PairsExt<'val, R> for Pairs<'val, R> {
                 }
                 Ok(pair)
             }
-            None => syntax_error(
-                "Expected at one token pair, got nothing!",
-                Position::Unknown,
-            ),
+            None => illegal_state("Expected at least one token pair, got nothing!"),
         }
     }
 }

partiql-parser/src/result.rs

@@ -190,6 +190,10 @@ pub enum ParserError {
     /// Indicates that there is a problem with run-time violation of some API.
     #[error("Invalid Argument: {message}")]
     InvalidArgument { message: String },
+
+    /// Indicates that there is an internal error that was not due to user input or API violation.
+    #[error("Illegal State: {message}")]
+    IllegalState { message: String },
 }
 
 impl ParserError {
@@ -209,6 +213,14 @@ impl ParserError {
             message: message.into(),
         }
     }
+
+    /// Convenience function to create a [`IllegalState`](ParserError::IllegalState).
+    #[inline]
+    pub fn illegal_state<S: Into<String>>(message: S) -> Self {
+        Self::IllegalState {
+            message: message.into(),
+        }
+    }
 }
 
 /// Convenience function to create an `Err([SyntaxError](ParserError::SyntaxError))`.
@@ -223,6 +235,12 @@ pub fn invalid_argument<T, S: Into<String>>(message: S) -> ParserResult<T> {
     Err(ParserError::invalid_argument(message))
 }
 
+/// Convenience function to create an `Err([IllegalState](ParserError::IllegalState))`.
+#[inline]
+pub fn illegal_state<T, S: Into<String>>(message: S) -> ParserResult<T> {
+    Err(ParserError::illegal_state(message))
+}
+
 impl<R> From<pest::error::Error<R>> for ParserError
 where
     R: fmt::Debug,