[Haskell-beginners] Parsing a file

26 Jan 2022

      Hello,

Warning: long post.

I've worked my way through various parsing tutorials using either Parsec
or ReadP.  I reached a point where I need to try parsing one of the
types of file for which I'm writing the parser.  I have written parsers
for various parts of this file:

eqatoms.out:
======================================================

Species :    1 (Si)
 atom    1 is equivalent to atom(s)
   1   2   3
 atom    2 is equivalent to atom(s)
   1   2   3
 atom    3 is equivalent to atom(s)
   1   2   3

Species :    2 (O)
 atom    1 is equivalent to atom(s)
   1   2   3   4   5   6
 atom    2 is equivalent to atom(s)
   1   2   3   4   5   6
 atom    3 is equivalent to atom(s)
   1   2   3   4   5   6
 atom    4 is equivalent to atom(s)
   1   2   3   4   5   6
 atom    5 is equivalent to atom(s)
   1   2   3   4   5   6
 atom    6 is equivalent to atom(s)
   1   2   3   4   5   6

======================================================

These are my imports:
======================================================
  import qualified Text.Parsec as Parsec

  import Text.Parsec (())

  import Control.Applicative

  import Control.Monad.Identity (Identity)
======================================================

These are my parsers:
======================================================
   :{
    species :: Parsec.Parsec String () (String,String)
    species = do
     --Parsec.char 'S'
     Parsec.string "Species"
     Parsec.spaces
     Parsec.char ':'
     Parsec.spaces
     digits <- Parsec.many1 Parsec.digit
     Parsec.spaces
     Parsec.char '('
     id <- Parsec.many1 Parsec.letter
     return (id,digits)
   :}

   :{
    atom = do
     Parsec.spaces
     Parsec.string "atom"
     Parsec.spaces
     digits <- Parsec.digit
     return digits
   :}

:{
  equivalents = do
   Parsec.spaces
   digits <- Parsec.digit
   return digits
:}
======================================================

Some simple tests:
======================================================
src = "oops"
Parsec.parse species src "Species :    1 (Si)"

Right ("Si","1")

src = "Parsing_File/eqatoms.out"
Parsec.parse atom src "atom    5 is equivalent to atom(s)"

Right '5'

src = "Parsing_File/eqatoms.out"
Parsec.parse (Parsec.many1 equivalents) src "   1   2   3   4   5   6"

: Right "123456"
======================================================

So, the individual parsers work as intended.  However, parsing an actual
file does not work.

I define a function to return the file contents:
======================================================
   :{
     input = do
       eqatoms <- readFile "Parsing_File/eqatoms.out"
       return eqatoms
   :}
======================================================

A test shows that my reader works:
======================================================
input

: Species :    1 (Si)\n atom    1 is equivalent to atom(s)\n   1   2
3\n atom    2 is equivalent to atom(s)\n   1   2   3\n atom    3 is
equivalent to atom(s)\n   1   2   3\n\nSpecies :    2 (O)\n atom    1 is
equivalent to atom(s)\n   1   2   3   4   5   6\n atom    2 is
equivalent to atom(s)\n   1   2   3   4   5   6\n atom    3 is
equivalent to atom(s)\n   1   2   3   4   5   6\n atom    4 is
equivalent to atom(s)\n   1   2   3   4   5   6\n atom    5 is
equivalent to atom(s)\n   1   2   3   4   5   6\n atom    6 is
equivalent to atom(s)\n   1   2   3   4   5   6\n

======================================================

I attempt to parse the input:
======================================================
   :{
     main = do
     eqatoms <- readFile "Parsing_File/eqatoms.out"
     Parsec.parse species "test species" eqatoms
     return
   :}

Prelude Parsec Text.Parsec Control.Applicative Control.Monad.Identity| Prelude Parsec Text.Parsec Control.Applicative Control.Monad.Identity| Prelude Parsec Text.Parsec Control.Applicative Control.Monad.Identity| Prelude Parsec Text.Parsec Control.Applicative Control.Monad.Identity| Prelude Parsec Text.Parsec Control.Applicative Control.Monad.Identity| 
<interactive>:250:3: error:
    ,* Couldn't match type `Either Parsec.ParseError' with `IO'
      Expected type: IO (String, String)
        Actual type: Either Parsec.ParseError (String, String)
    ,* In a stmt of a 'do' block:
        Parsec.parse species "test species" eqatoms
      In the expression:
        do eqatoms <- readFile "Parsing_File/eqatoms.out"
           Parsec.parse species "test species" eqatoms
           return
      In an equation for `main':
          main
            = do eqatoms <- readFile "Parsing_File/eqatoms.out"
                 Parsec.parse species "test species" eqatoms
                 return

<interactive>:251:3: error:
    ,* Couldn't match expected type `IO b'
                  with actual type `a0 -> m0 a0'
    ,* Probable cause: `return' is applied to too few arguments
      In a stmt of a 'do' block: return
      In the expression:
        do eqatoms <- readFile "Parsing_File/eqatoms.out"
           Parsec.parse species "test species" eqatoms
           return
      In an equation for `main':
          main
            = do eqatoms <- readFile "Parsing_File/eqatoms.out"
                 Parsec.parse species "test species" eqatoms
                 return
    ,* Relevant bindings include
        main :: IO b (bound at <interactive>:248:3)
======================================================

Can someone please help me to get this to work?

Thanks for reading to the end of this very long post.
Roger

[Haskell-beginners] Parsing a file

Roger Mason