core: runFirstIO

src/Parser/Interface.md

@@ -19,10 +19,9 @@ Combine the parser state at time of error with an error message.
 
 ```idris
 public export
-record ParseError where
-  constructor MkParseError
-  state : ParserInternal Id
-  message : String
+data ParseError : Type where
+  MkParseError : (state : ParserInternal Id) -> (message : String) -> ParseError
+  BeforeParse : (message : String) -> ParseError
 ```
 
 <!-- idris
@@ -33,6 +32,8 @@ Show ParseError where
         (line, col) = (show line, show col)
         position = show state.position.index
     in "Error at line \{line}, column \{col} (\{position}): \{message}"
+  show (BeforeParse message) =
+    "Error before parsing: \{message}"
 -->
 
 ## Type Alias
@@ -95,6 +96,17 @@ rundownFirst f =
   runExcept . guardMaybe "No returning parses" . runChoose {f = Maybe} $ f
 ```
 
+Provide wrappers for `rundownFirst` for evaluating it in various contexts.
+
+```idris
+export
+runFirstIO : (f : Parser a) -> String -> IO (Either ParseError a)
+runFirstIO f str = do
+  Just state <- newInternalIO str
+    | _ => pure . Left $ BeforeParse "Empty input"
+  runEff (rundownFirst f) [handleParserStateIO state]
+```
+
 ## Utility functionality
 
 ### Parser combinators