runParser function that receives initial SourcePos about megaparsec

[mrkkrp]

Well, it can be implemented on lower level if we place it in Text.Megaparsec.Prim module. When you run parser, it constructs initial state, which includes position, we could write a bit more general version of runParserT that takes the position as argument instead.

Since you open this sort of issue, you probably can explain how this feature would be widely useful in practice?

[chamini2]

Well, I'm currently writing an interpreter for a functional programming language that supports mixfix operators, these are operators where the arguments can be in any part of them, I'll illustrate with an example:

_+_ : Nat -> Nat -> Nat
Z + n     = n
(S m) + n = S (m + n)

So we define the operator, indicating where the arguments go with the underscore character _, a more complex example is if_then_else_

if_then_else_ : forall x : Type . Bool -> x -> x -> x
if True  then y else _ = y
if False then _ else n = n

And we have a statement for indicating the fixity and precedence

operator if_then_else_ prefix 1
operator _+_           infixL 4

So I first parse the source of a program with Alex and Happy, and run over the Abstract Syntax Tree to get such operator statements, creating rules for a dynamic parser, in Megaparsec.

So in every expression found in the AST, I must run a parser, but just on that part, or at least that's how I'm doing it.

For example,

x : Bool -> Nat
x b = S Z + (if b then Z else S Z)

I want the function definition to become

x b = _+_ (S Z) (if_then_else_ b Z (S Z))

I don't know if this classifies as widely used, but I thought more people may want to parse specific parts of some already parsed information, not starting at the position (1, 1) of the information.

[mrkkrp]

I think this (as well as ability to get rest of input on success and failure) will be added in version 4.2.0, perhaps.