castingsmlsmlnjmlton

How to cast value in SML? Trying to take return type of S-Expression parser and convert to data structure

I am using the S-Expression library included in SML/NJ. In my use case, my file containing the SExpressions will always be of type String List List List. The SExpParser.parse successfully parses my file. The problem is the return type of the parser is SExp.value List, where value is defined as

datatype value
      = SYMBOL of Atom.atom
      | BOOL of bool
      | INT of IntInf.int
      | FLOAT of real
      | STRING of string
      | QUOTE of value
      | LIST of value list

My function to convert the data into a graph is fun makeGraph (n:string list list list). Now, the problem is, naturally, the compiler yells at me because it cannot determine at compile time that the return type of the parser will actually be a string list list list. So, I have tried using pattern matching to determine the type, but I keep failing to get it to compile (something off of this idea).

Some things I have tried that have not worked:

fun convert (SExp.LIST ((SExp.LIST ((SExp.STRING s)::ss))::ls)) = ArrayGraph.makeGraph ls 
  | convert _ = raise Fail "convert"

fun convert values:SExp.value =
   case values of
      SExp.LIST ((SExp.LIST ((SExp.STRING s)::ss))::ls) => ArrayGraph.makeGraph ls 
    | _ => raise Fail "convert" 

fun convert values:SExp.value =
   case values of
      SExp.LIST(v) => map (fn x => convert x) v 
    | SExp.STRING(s) => s::nil
    | _ => raise Fail "convert"

I am really stumped on this problem and would appreciate any help. I also do not seem to find any examples of people using this library, so if you have any of those, I would appreciate a link. Thanks.

For reference, here is a link to the sml/nj s-expression code: https://github.com/smlnj/smlnj/blob/main/smlnj-lib/SExp/README

Solution

  • The first two can't work because ls is a SExp.value list, not a string list list.

    The third can't work because s::nil is a string list, and if convert x is a string list list, then map (fn x => convert x) v is a string list list list.

    Start at the bottom, with the strings, and work your way upwards over each nesting list.

    Something like this should work:

    
fun to_string (SExp.STRING s) = s
  | to_string _ = raise Fail "to string"

fun to_string_list (SExp.LIST ss) = map to_string ss
  | to_string_list  _ = raise Fail "to string list"

fun to_string_list_list (SExp.LIST ss) = map to_string_list ss
  | to_string_list_list  _ = raise Fail "to string list list"

fun convert sexp = ArrayGraph.makeGraph (to_string_list_list sexp)