Databases
StackOverflow2013
Postsdbo
POSimple interpreter written in Haskell, saves up print output until the end, instead of when it comes across a print statement
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POSimple interpreter written in Haskell, saves up print output until the end, instead of when it comes across a print statement
    primarykey
    data
    text
    <p>Below is my attempt at a very simple interpreter which is translated from the Java version of the program described in Chapter 1 of "modern compiler implementation in Java" by Andrew w. Appel, and operates directly upon a tree which represents the language.</p> <p>Basically, my problem is that it saves up all the output until the end before anything at all is printed. I'm really just looking for advice on how to restructure it so that the "print" statements are printed as they are interpreted.</p> <pre><code>module Interpreter where -------------------------------------------------------------------- type Id = [Char] type Output = [Char] type Value = Int type Table = [(Id, Value)] data Stm = CompoundStm Stm Stm | AssignStm Id Exp | PrintStm ExpList deriving Show data Exp = IdExp Id | NumExp Value | OpExp Exp Op Exp | EseqExp Stm Exp deriving Show data ExpList = PairExpList Exp ExpList | LastExpList Exp deriving Show data Op = Plus | Minus | Times | Div deriving Show -------------------------------------------------------------------- example :: Stm example = CompoundStm (AssignStm "a" (OpExp (NumExp 5) Plus (NumExp 3))) (CompoundStm (AssignStm "b" (EseqExp (PrintStm (PairExpList (IdExp "a") (LastExpList (OpExp (IdExp "a") Minus (NumExp 1))))) (OpExp (NumExp 10) Times (IdExp "a")))) (PrintStm (LastExpList (IdExp "b")))) -------------------------------------------------------------------- tableUpdate :: Table -&gt; Id -&gt; Value -&gt; Table tableUpdate t i v = (i,v):t tableLookup :: Table -&gt; Id -&gt; Value tableLookup ((x,v):t) i | x == i = v tableLookup ((x,v):t) i | x /= i = tableLookup t i -------------------------------------------------------------------- execute :: Stm -&gt; IO() execute s = putStr ((\(o,_)-&gt;o) (interpStm s ("",[]))) interpStm :: Stm -&gt; (Output, Table) -&gt; (Output, Table) interpStm (CompoundStm l r) ot = interpStm r (interpStm l ot) interpStm (PrintStm el) (o,t) = (interpExpList el o t) interpStm (AssignStm i e) (o,t) = f i (interpExp e (o,t)) where f i (v,o,t) = (o, tableUpdate t i v) interpExp :: Exp -&gt; (Output, Table) -&gt; (Value, Output, Table) interpExp (IdExp i) (o,t) = (tableLookup t i, o, t) interpExp (NumExp v) (o,t) = (v, o, t) interpExp (EseqExp s e) ot = interpExp e (interpStm s ot) interpExp (OpExp l op r) (o,t) = f (interpExp l (o,t)) op r where f (v,o,t) op r = g v op (interpExp r (o,t)) g v1 Plus (v2,o,t) = (v1 + v2, o, t) g v1 Minus (v2,o,t) = (v1 - v2, o, t) g v1 Times (v2,o,t) = (v1 * v2, o, t) g v1 Div (v2,o,t) = (v1 `div` v2, o, t) interpExpList :: ExpList -&gt; Output -&gt; Table -&gt; (Output, Table) interpExpList (LastExpList e) o t = f (interpExp e (o,t)) where f (v, o, t) = (o ++ (show v) ++ "\n", t) interpExpList (PairExpList e el) o t = f (interpExp e (o,t)) where f (v, o, t) = interpExpList el (o ++ (show v) ++ " ") t </code></pre>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USDMK
    1. USTom Busby
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POSimple interpreter written in Haskell, saves up print output until the end, instead of when it comes across a print statement
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POSimple interpreter written in Haskell, saves up print output until the end, instead of when it comes across a print statement
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POSimple interpreter written in Haskell, saves up print output until the end, instead of when it comes across a print statement
      1. This table or related slice is empty.
      1. VTUpMod
    1. COYou would have to make use a combined monad of StateT (for the Table) and IO for interactive printing (note, IO cannot be a transformer) . If this sounds like gobble-de-gook, then I wouldn't worry about trying to interactively print output just yet and first consider the simpler problem of using a State monad for plumbing the updateable Table through your code.
      singulars
      1. POSimple interpreter written in Haskell, saves up print output until the end, instead of when it comes across a print statement
      1. USstephen tetley
    2. COI did an advanced functional programming module on my course so I have used the State monad before, but I still don't completely understand how it works, so I'd struggle to build a combined IO/ST monad, do you know where I could have a proper read-up on the state monad? I'll see about rewriting Table as a ST monad to begin with though
      singulars
      1. POSimple interpreter written in Haskell, saves up print output until the end, instead of when it comes across a print statement
      1. USTom Busby
    3. COA state monad is a good idea for readability and reducing the likelihood of bugs, but it's certainly not necessary. Simply converting your code to IO will work.
      singulars
      1. POSimple interpreter written in Haskell, saves up print output until the end, instead of when it comes across a print statement
      1. USHeatsink
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload