translate Haskell (monadic) to F# - haskell

Question

Help me translate following block of the Haskell code. The run function produces text string that corresponding to a given regex that abstracted as Pattern.
Declaration of the type Pattern you can see below in the block of F# code. You can test run function like
genex $ POr [PConcat [PEscape( DoPa 1) 'd'], PConcat [PEscape (DoPa 2) 'd']]
{-# LANGUAGE RecordWildCards, NamedFieldPuns #-}
import qualified Data.Text as T
import qualified Control.Monad.Stream as Stream
import Text.Regex.TDFA.Pattern
import Control.Monad.State
import Control.Applicative
genex = Stream.toList . run
maxRepeat :: Int
maxRepeat = 3
each = foldl1 (<|>) . map return
run :: Pattern -> Stream.Stream T.Text
run p = case p of
PBound low high p -> do
n <- each [low..maybe (low+maxRepeat) id high]
fmap T.concat . sequence $ replicate n (run p)
PConcat ps -> fmap T.concat . Stream.suspended . sequence $ map run ps
POr xs -> foldl1 mplus $ map run xs
PEscape {..} -> case getPatternChar of
'd' -> chars $ ['0'..'9']
'w' -> chars $ ['0'..'9'] ++ '_' : ['a'..'z'] ++ ['A'..'Z']
ch -> isChar ch
_ -> error $ show p
where
isChar = return . T.singleton
chars = each . map T.singleton
Below I give my poor attempt. It works but incorrectly. The problem is in the following.
Let assume parse produces Pattern like that
parse "\\d\\d";;
val it : Pattern = POr [PConcat [PEscape (DoPa 1,'d'); PEscape (DoPa 2,'d')]]
and
parse "\\d{2}";;
val it : Pattern = POr [PConcat [PBound (2,Some 2,PEscape (DoPa 1,'d'))]]
So feeding both patterns to run I expect to receive seq [['2'; '2']; ['2'; '3']; ['2'; '1']; ['2'; '4']; ...] that corresponding to seq ["22"; "23"; "21"; "24"; ...] (2 symbols per string)
This is valid in the first case,
POr [PConcat [PEscape (DoPa 1,'d'); PEscape (DoPa 2,'d')]] |> run;;
val it : seq = seq [['2'; '2']; ['2'; '3']; ['2'; '1']; ['2'; '4']; ...]
seq ["22"; "23"; "21"; "24"; ...]
but not in the second
POr [PConcat [PBound (2,Some 2,PEscape (DoPa 1,'d'))]] |> run;;
val it : seq = seq [['2']; ['2']; ['2']; ['3']; ...]
seq ["2"; "2", "2"; "3", "2"; "1", "2"; "4";...] (1 symbol per string)
I tested different variants with the following clauses:
| POr ps -> Seq.concat (List.map run ps)
| PConcat ps -> (sequence (List.map (run >> Seq.concat) ps))
| PBound (low,high,p) ->
but all in vain. I can't figure out the valid translation.
-Maybe I should use String or Array instead of char list.
-And I assume that Seq is quite good analogue to Control.Monad.Stream. Is it right?
Thanks in advance for help
open System
/// Used to track elements of the pattern that accept characters or are anchors
type DoPa = DoPa of int
/// Pattern is the type returned by the regular expression parser.
/// This is consumed by the CorePattern module and the tender leaves
/// are nibbled by the TNFA module.
type Pattern = PEmpty
| POr of Pattern list // flattened by starTrans
| PConcat of Pattern list // flattened by starTrans
| PBound of int * (int option) * Pattern // eliminated by starTrans
| PEscape of DoPa * char // Backslashed Character
let maxRepeat = 3
let maybe deflt f opt =
match opt with
| None -> deflt
| Some v -> f v
/// Cartesian production
/// try in F# interactive: sequence [[1;2];[3;4]];;
let rec sequence = function
| [] -> Seq.singleton []
| (l::ls) -> seq { for x in l do for xs in sequence ls do yield (x::xs) }
let from'space'to'tilda = [' '..'~'] |> List.ofSeq
let numbers = ['0'..'9'] |> List.ofSeq
let numbers'and'alphas = (['0'..'9'] # '_' :: ['a'..'z'] # ['A'..'Z']) |> List.ofSeq
let whites = ['\009'; '\010'; '\012'; '\013'; '\032' ] |> List.ofSeq
let rec run (p:Pattern) : seq<char list> =
let chars chs = seq { yield [for s in chs -> s] }
match p with
| POr ps -> Seq.concat (List.map run ps)
| PConcat ps -> (sequence (List.map (run >> Seq.concat) ps))
| PBound (low,high,p) ->
let ns = seq {low .. maybe (low + maxRepeat) id high}
Seq.concat (seq { for n in ns do yield sequence (List.replicate n (((run >> Seq.concat) p))) })
// Seq.concat (seq { for n in ns do yield ((List.replicate n (run p)) |> Seq.concat |> List.ofSeq |> sequence)})
//((List.replicate low (run p)) |> Seq.concat |> List.ofSeq |> sequence)
// PConcat [ for n in ns -> p] |> run
| PEscape(_, ch) ->
match ch with
| 'd' -> chars numbers
| 'w' -> chars numbers'and'alphas
| ch -> chars [ch]
| _ -> Seq.empty

Answer 1

I don't know why you didn't translate Data.Text from Haskell to string in F#, you just need to mimic two functions. Apart from that I did just a few changes to make it work, this way you can compare it easily with your original code, see replaced code between (* *)
open System
// Mimic Data.Text as T
module T =
let concat (x:seq<_>) = System.String.Concat x
let singleton (x:char) = string x
/// Used to track elements of the pattern that accept characters or are anchors
type DoPa = DoPa of int
/// Pattern is the type returned by the regular expression parser.
/// This is consumed by the CorePattern module and the tender leaves
/// are nibbled by the TNFA module.
type Pattern = PEmpty
| POr of Pattern list // flattened by starTrans
| PConcat of Pattern list // flattened by starTrans
| PBound of int * (int option) * Pattern // eliminated by starTrans
| PEscape of DoPa * char // Backslashed Character
let maxRepeat = 3
let maybe deflt f opt =
match opt with
| None -> deflt
| Some v -> f v
/// Cartesian production
/// try in F# interactive: sequence [[1;2];[3;4]];;
let rec sequence = function
| [] -> Seq.singleton []
| (l::ls) -> seq { for x in l do for xs in sequence ls do yield (x::xs) }
let from'space'to'tilda = [' '..'~'] |> List.ofSeq
let numbers = ['0'..'9'] |> List.ofSeq
let numbers'and'alphas = (['0'..'9'] # '_' :: ['a'..'z'] # ['A'..'Z']) |> List.ofSeq
let whites = ['\009'; '\010'; '\012'; '\013'; '\032' ] |> List.ofSeq
let rec run (p:Pattern) (*: seq<char list> *) =
(* let chars chs = seq { yield [for s in chs -> s] } *)
let chars (chs:seq<char>) = Seq.map string chs
match p with
| POr ps -> Seq.concat (List.map run ps)
| PConcat ps -> Seq.map T.concat << sequence <| List.map run ps (* (sequence (List.map (run >> Seq.concat) ps)) *)
| PBound (low,high,p) ->
seq {
for n in [low..maybe (low+maxRepeat) id high] do
yield! ( (Seq.map T.concat << sequence) (List.replicate n (run p)) )}
(*let ns = seq {low .. maybe (low + maxRepeat) id high}
Seq.concat (seq { for n in ns do yield sequence (List.replicate n (((run >> Seq.concat) p))) *)
// Seq.concat (seq { for n in ns do yield ((List.replicate n (run p)) |> Seq.concat |> List.ofSeq |> sequence)})
//((List.replicate low (run p)) |> Seq.concat |> List.ofSeq |> sequence)
// PConcat [ for n in ns -> p] |> run
| PEscape(_, ch) ->
match ch with
| 'd' -> chars numbers
| 'w' -> chars numbers'and'alphas
| ch -> chars [ch]
| _ -> Seq.empty
UPDATE
If you are translating Haskell code to F# you may try using this code which mimics many Haskell functions, including those using Type Classes.
I did a test translating as close as possible to your original Haskell code but using F# List (not lazy) and looks like this:
#load "Prelude.fs"
#load "Monad.fs"
#load "Applicative.fs"
#load "Monoid.fs"
open Prelude
open Control.Monad.Base
open Control.Applicative
module T =
let concat (x:list<_>) = System.String.Concat x
let singleton (x:char) = string x
type DoPa = DoPa of int
type Pattern = PEmpty
| POr of Pattern list
| PConcat of Pattern list
| PBound of int * (int option) * Pattern
| PEscape of DoPa * char
let maxRepeat = 3
let inline each x = foldl1 (<|>) << map return' <| x
let rec run p:list<_> =
let inline isChar x = return' << T.singleton <| x
let inline chars x = each << map T.singleton <| x
match p with
| PBound (low,high,p) -> do' {
let! n = each [low..maybe (low+maxRepeat) id high]
return! (fmap T.concat << sequence <| replicate n (run p))}
| PConcat ps -> fmap T.concat << sequence <| map run ps
| POr xs -> foldl1 mplus <| map run xs
| PEscape (_, ch) ->
match ch with
| 'd' -> chars <| ['0'..'9']
| 'w' -> chars <| ['0'..'9'] # '_' :: ['a'..'z'] # ['A'..'Z']
| ch -> isChar ch
| _ -> failwith <| string p
let genex = run