[
arXiv:1603.08865]
Abstract
This article is about an implementation and compilation technique that is used in RAW-Feldspar which is a complete rewrite of the Feldspar embedded domain-specific language (EDSL) (Axelsson et al. 2010). Feldspar is high-level functional language that generates efficient C code to run on embedded targets.
The gist of the technique presented in this post is the following: rather writing a back end that converts pure Feldspar expressions directly to C, we translate them to a low-level monadic EDSL. From the low-level EDSL, C code is then generated.
This approach has several advantages:
- The translation is simpler to write than a complete C back end.
- The translation is between two typed EDSLs, which rules out many potential errors.
- The low-level EDSL is reusable and can be shared between several high-level EDSLs.
Although the article contains a lot of code, most of it is in fact reusable. As mentioned in Discussion, we can write the same implementation in less than 50 lines of code using generic libraries that we have developed to support Feldspar.
Introduction
There has been a recent trend to implement low-level imperative EDSLs based on monads (Persson, Axelsson, and Svenningsson 2012; J. D. Svenningsson and Svensson 2013; Hickey et al. 2014; Bracker and Gill 2014). By using a deep embedding of monadic programs, imperative EDSL code can be translated directly to corresponding code in the back end (e.g. C or JavaScript code).
A deep embedding of a monad with primitive instruction gives a representation of the statements of an imperative language. But in order for a language to be useful, there also needs to be a notion of pure expressions. Take the following program as example:
prog = do
i <- readInput
writeOutput (i+i)
In order to generate code from this program we need a representation of the whole syntax, including the expression i+i. And it is usually not enough to support such simple expressions. For example, in RAW-Feldspar, we can write the following,
prog = do
i <- fget stdin
printf "sum: %d\n" $ sum $ map square (0 ... i)
where
square x = x*x
and have it generate this C code:
#include <stdint.h>
#include <stdio.h>
int main()
{
uint32_t v0;
uint32_t let1;
uint32_t state2;
uint32_t v3;
fscanf(stdin, "%u", &v0);
let1 = v0 + 1;
state2 = 0;
for (v3 = 0; v3 < (uint32_t) (0 < let1) * let1; v3++) {
state2 = v3 * v3 + state2;
}
fprintf(stdout, "sum: %d\n", state2);
return 0;
}
Note that the pure expression sum $ map square (0 ... i) was turned into several statements in the C code (everything from let1 = to the end of the loop). What happened here was that sum $ map square (0 ... i) was first transformed to a more low-level (but still pure) expression that involves a let binding and a looping construct. This expression was then translated to C code.
However, the story is actually more interesting than that: Instead of translating the pure expression directly to C, which can be a tedious and error-prone task, it is first translated to a typed low-level EDSL that only has simple expressions and a straightforward translation to C. Going through a typed EDSL makes the translation simple and robust, and this low-level EDSL has the advantage of being generic and reusable between different high-level EDSLs.
This post will show a simplified version of the technique used in RAW-Feldspar. For the full story, see the RAW-Feldspar implementation (tag article-2016-03). The low-level EDSL that is used for compiling Feldspar to C is provided by the package imperative-edsl.
Outline of the technique
Our technique is based on a deep monadic embedding of imperative programs:
data Prog exp a where ...
instance Monad (Prog exp)
By parameterizing Prog on the expression type exp, we can have imperative programs with different representations of expressions.
Next, we define two expression languages:
data LowExp a where ...
data HighExp a where ...
LowExp is a simple language containing only variables, literals, operators and function calls. HighExp is a richer language that may contain things like let binding, tuples and pure looping constructs.
Since LowExp is so simple, we can easily – once and for all – write a back end that translates Prog LowExp to C code:
lowToCode :: Prog LowExp a -> String
Once this is done, we can implement the compiler for HighExp simply as a typed translation function between two EDSLs:
translateHigh :: Prog HighExp a -> Prog LowExp a
compile :: Prog HighExp a -> String
compile = lowToCode . translateHigh
If we want to make changes to HighExp, we only need to update the translateHigh function. And if we want to implement an EDSL with a different expression language, we simply write a different translation function for that language.
A deep embedding of imperative programs
The code is available as a literate Haskell file.
We make use of the following GHC extensions:
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE UndecidableInstances #-}
And we also need a few helper modules:
import Control.Monad.State
import Control.Monad.Writer
import Data.Int
import Data.IORef
We are now ready to define our monadic deep embedding. Our EDSL will be centered around the monad Prog (inspired by the Program monad in Operational and the work of J. D. Svenningsson and Svensson (2013)):
data Prog exp a where
Return :: a -> Prog exp a
(:>>=) :: Prog exp a -> (a -> Prog exp b) -> Prog exp b
CMD :: CMD exp a -> Prog exp a
instance Functor (Prog exp) where
fmap f m = m >>= return . f
instance Applicative (Prog exp) where
pure = return
(<*>) = ap
instance Monad (Prog exp) where
return = Return
(>>=) = (:>>=)
As seen in the Monad instance, Return and (:>>=) directly represent uses of return and (>>=), respectively. CMD injects a primitive instruction from the type CMD a which will be defined below.
The attentive reader may react that Prog actually does not satisfy the monad laws, since it allows us to observe the nesting of (>>=) in a program. This is, however, not a problem in practice: we will only interpret Prog in ways that are ignorant of the nesting of (>>=).
(The Operational package honors the monad laws by only providing an abstract interface to Program, preventing interpretations that would otherwise be able to observe the nesting of (>>=).)
Primitive instructions
The CMD type contains the primitive instructions of the EDSL. It has instructions for mutable references, input/output and loops:
data CMD exp a where
-- References:
InitRef :: Type a => exp a -> CMD exp (Ref a)
GetRef :: Type a => Ref a -> CMD exp (Val a)
SetRef :: Type a => Ref a -> exp a -> CMD exp ()
-- Input/output:
Read :: CMD exp (Val Int32)
Write :: exp Int32 -> CMD exp ()
PrintStr :: String -> CMD exp ()
-- Loops:
For :: exp Int32 -> (Val Int32 -> Prog exp ()) -> CMD exp ()
The instructions InitRef, GetRef and SetRef correspond to newIORef, readIORef and writeIORef from Data.IORef.
Read and Write are used to read integers from stdin and write integers to stdout. PrintStr prints a static string to stdout.
For n body represents a for-loop of n iterations, where body gives the program to run in each iteration. The argument to body ranges from 0 to n-1.
As explained in Outline of the technique, exp represents pure expressions, and we have it as a parameter in order to be able to use the same instructions with different expression types.
Now we have three things left to explain: Type, Val and Ref.
The Type class is just used to restrict the set of types that can be used in our EDSL. For simplicity, we will go with the following class
class (Eq a, Ord a, Show a) => Type a
instance (Eq a, Ord a, Show a) => Type a
Val is used whenever an instruction produces a pure value (e.g. GetRef). It would make sense to use exp a instead of Val a. However, that would lead to problems later when we want to translate between different expression types. We can think of Val a as representing a value in any expression type.
In order to interpret a program, we must have concrete representations of the types returned by the primitive instructions. For example, interpreting the program CMD Read :>>= \a -> ..., requires creating an actual value of type Val Int32 to pass to the continuation. However, depending on the interpretation, we may need different representations for this value.
In a standard interpretation (e.g. running the program in Haskell’s IO monad), we want Val a to be represented by an actual value of type a. But in a non-standard interpretation such as compilation to C code, we want Val a to be a variable identifier. To reconcile these different needs, we define Val as a sum type:
data Val a
= ValRun a -- Concrete value
| ValComp VarId -- Symbolic value
-- Variable identifier
type VarId = String
For the same reason, Ref is defined to be either an actual IORef or a variable identifier:
data Ref a
= RefRun (IORef a) -- Concrete reference
| RefComp VarId -- Symbolic reference
Unfortunately, having sum types means that our interpretations will sometimes have to do incomplete pattern matching. For example, the standard interpretation of GetRef r will assume that r is a RefRun (a variable identifier has no meaning in the standard interpretation), while the interpretation that compiles to C will assume that it is a RefComp. However, the EDSL user is shielded from this unsafety. By making Val and Ref abstract types, we ensure that users’ programs are unaware of the interpretation which is applied to them. As long as our interpreters are correct, no-one will suffer from the unsafety due to Val and Ref being sum types.
Interpretation
The following function lifts an interpretation of instructions in a monad m to an interpretation of programs in the same monad (corresponding to interpretWithMonad in Operational):
interpret :: Monad m
=> (forall a . CMD exp a -> m a)
-> Prog exp a -> m a
interpret int (Return a) = return a
interpret int (p :>>= k) = interpret int p >>= interpret int . k
interpret int (CMD cmd) = int cmd
Using interpret, we can define the standard interpretation that runs a program in the IO monad:
runIO :: EvalExp exp => Prog exp a -> IO a
runIO = interpret runCMD
runCMD :: EvalExp exp => CMD exp a -> IO a
runCMD (InitRef a) = RefRun <$> newIORef (evalExp a)
runCMD (GetRef (RefRun r)) = ValRun <$> readIORef r
runCMD (SetRef (RefRun r) a) = writeIORef r (evalExp a)
runCMD Read = ValRun . read <$> getLine
runCMD (Write a) = putStr $ show $ evalExp a
runCMD (PrintStr s) = putStr s
runCMD (For n body) =
mapM_ (runIO . body . ValRun) [0 .. evalExp n - 1]
Most of the work is done in runCMD which gives the interpretation of each instruction. Note the use of ValRun and RefRun on both sides of the equations. As said earlier, ValComp and RefComp are not used in the standard interpretation.
Since running a program also involves evaluating expressions, we need the constraint EvalExp exp that gives access to the function evalExp (for example in the interpretation of InitRef). EvalExp is defined as follows:
class EvalExp exp where
-- Evaluate a closed expression
evalExp :: exp a -> a
Front end
We are now ready to define the smart constructors that we will give as the interface to the EDSL user. For many instructions, the smart constructor just takes care of lifting it to a program using CMD, for example:
initRef :: Type a => exp a -> Prog exp (Ref a)
initRef = CMD . InitRef
setRef :: Type a => Ref a -> exp a -> Prog exp ()
setRef r a = CMD (SetRef r a)
But instructions that involve Val will need more care. We do not actually want to expose Val to the user, but rather use exp to hold values. To achieve this, we introduce a class for expressions supporting injection of constants and variables:
class FreeExp exp where
-- Inject a constant value
constExp :: Type a => a -> exp a
-- Inject a variable
varExp :: Type a => VarId -> exp a
Using FreeExp, we can make a general function that converts Val to an expression – independent of interpretation:
valToExp :: (Type a, FreeExp exp) => Val a -> exp a
valToExp (ValRun a) = constExp a
valToExp (ValComp v) = varExp v
Now we can define the rest of the front end, using valToExp where needed:
getRef :: (Type a, FreeExp exp) => Ref a -> Prog exp (exp a)
getRef = fmap valToExp . CMD . GetRef
readInput :: FreeExp exp => Prog exp (exp Int32)
readInput = valToExp <$> CMD Read
writeOutput :: exp Int32 -> Prog exp ()
writeOutput = CMD . Write
printStr :: String -> Prog exp ()
printStr = CMD . PrintStr
for :: FreeExp exp
=> exp Int32 -> (exp Int32 -> Prog exp ()) -> Prog exp ()
for n body = CMD $ For n (body . valToExp)
As a convenience, we also provide a function for modifying a reference using a pure function:
modifyRef :: (Type a, FreeExp exp)
=> Ref a -> (exp a -> exp a) -> Prog exp ()
modifyRef r f = setRef r . f =<< getRef r
Pure expressions
Now that we have a front end for the imperative EDSL, we also need to define an expression type before we can use it for anything.
We begin with a very simple expression type that only has variables, literals and some primitive functions:
data LowExp a where
LVar :: Type a => VarId -> LowExp a
LLit :: Type a => a -> LowExp a
LAdd :: (Num a, Type a) => LowExp a -> LowExp a -> LowExp a
LMul :: (Num a, Type a) => LowExp a -> LowExp a -> LowExp a
LNot :: LowExp Bool -> LowExp Bool
LEq :: Type a => LowExp a -> LowExp a -> LowExp Bool
As common in Haskell EDSLs, we provide a Num instance to make it easier to construct expressions:
instance (Num a, Type a) => Num (LowExp a) where
fromInteger = LLit . fromInteger
(+) = LAdd
(*) = LMul
This instance allows us to write things like 5+6 :: LowExp Int32.
The implementation of LowExp is completed by instantiating the EvalExp and FreeExp classes:
instance EvalExp LowExp where
evalExp (LLit a) = a
evalExp (LAdd a b) = evalExp a + evalExp b
evalExp (LMul a b) = evalExp a * evalExp b
evalExp (LNot a) = not $ evalExp a
evalExp (LEq a b) = evalExp a == evalExp b
instance FreeExp LowExp where
constExp = LLit
varExp = LVar
First example
We can now write our first imperative example program:
sumInput :: Prog LowExp ()
sumInput = do
r <- initRef 0
printStr "Please enter 4 numbers\n"
for 4 $ \i -> do
printStr " > "
n <- readInput
modifyRef r (+n)
printStr "The sum of your numbers is "
s <- getRef r
writeOutput s
printStr ".\n"
This program uses a for-loop to read four number from the user. It keeps the running sum in a reference, and prints the final sum before quitting. An example run can look as follows:
*EDSL_Compilation> runIO sumInput
Please enter 4 numbers
> 1
> 2
> 3
> 4
The sum of your numbers is 10.
Code generation
The code generator is defined using interpret with the target being a code generation monad Code:
code :: ShowExp exp => Prog exp a -> Code a
code = interpret codeCMD
lowToCode :: Prog LowExp a -> String
lowToCode = runCode . code
The definition of codeCMD is deferred to Appendix: Code generation.
In this article, the code generator only produces simple imperative pseudo-code. Here is the code generated from the sumInput example:
*EDSL_Compilation> putStrLn $ lowToCode sumInput
r0 <- initRef 0
readInput "Please enter 4 numbers\n"
for v1 < 4
readInput " > "
v2 <- stdin
v3 <- getRef r0
setRef r0 (v3 + v2)
end for
readInput "The sum of your numbers is "
v4 <- getRef r0
writeOutput v4
readInput ".\n"
High-level expressions
The expression language LowExp is really too simple to be very useful. For example, it does not allow any kind of iteration, so any iterative program has to be written using the monadic for-loop.
To fix this, we introduce a new expression language, HighExp:
data HighExp a where
-- Simple constructs (same as in LowExp):
HVar :: Type a => VarId -> HighExp a
HLit :: Type a => a -> HighExp a
HAdd :: (Num a, Type a) => HighExp a -> HighExp a -> HighExp a
HMul :: (Num a, Type a) => HighExp a -> HighExp a -> HighExp a
HNot :: HighExp Bool -> HighExp Bool
HEq :: Type a => HighExp a -> HighExp a -> HighExp Bool
-- Let binding:
Let :: Type a
=> HighExp a -- value to share
-> (HighExp a -> HighExp b) -- body
-> HighExp b
-- Pure iteration:
Iter :: Type s
=> HighExp Int32 -- number of iterations
-> HighExp s -- initial state
-> (HighExp s -> HighExp s) -- step function
-> HighExp s -- final state
instance (Num a, Type a) => Num (HighExp a) where
fromInteger = HLit . fromInteger
(+) = HAdd
(*) = HMul
instance FreeExp HighExp where
constExp = HLit
varExp = HVar
HighExp contains the same simple constructs as LowExp, but has been extended with two new constructs: let binding and iteration. Both of these constructs use higher-order abstract syntax (HOAS) (Pfenning and Elliot 1988) as a convenient way to introduce local variables.
The following program uses HighExp to represent expressions:
powerInput :: Prog HighExp ()
powerInput = do
printStr "Please enter two numbers\n"
printStr " > "; m <- readInput
printStr " > "; n <- readInput
printStr "Here's a fact: "
writeOutput m; printStr "^"; writeOutput n; printStr " = "
writeOutput $ Iter n 1 (*m)
printStr ".\n"
It asks the user to input two numbers m and n, and and prints the result of m^n. Note the use of Iter to compute m^n as a pure expression.
Compilation as a typed EDSL translation
Now we are getting close to the main point of the article. This is what we have so far:
- A way to generate code from low-level programs (of type
Prog LowExp a).
- The ability to write high-level programs (of type
Prog HighExp a).
What is missing is a function that translates high-level programs to low-level programs. Generalizing the problem a bit, we need a way to rewrite a program over one expression type to a program over a different expression type – a process which we call “re-expressing”.
Since Prog is a monad like any other, we can actually express this translation function using interpret:
reexpress :: (forall a . exp1 a -> Prog exp2 (exp2 a))
-> Prog exp1 b
-> Prog exp2 b
reexpress reexp = interpret (reexpressCMD reexp)
reexpressCMD :: (forall a . exp1 a -> Prog exp2 (exp2 a))
-> CMD exp1 b
-> Prog exp2 b
reexpressCMD reexp (InitRef a) = CMD . InitRef =<< reexp a
reexpressCMD reexp (GetRef r) = CMD (GetRef r)
reexpressCMD reexp (SetRef r a) = CMD . SetRef r =<< reexp a
reexpressCMD reexp Read = CMD Read
reexpressCMD reexp (Write a) = CMD . Write =<< reexp a
reexpressCMD reexp (PrintStr s) = CMD (PrintStr s)
reexpressCMD reexp (For n body) = do
n' <- reexp n
CMD $ For n' (reexpress reexp . body)
The first argument to reexpress translates an expression in the source language to an expression in the target language. Note the type of this function: exp1 a -> Prog exp2 (exp2 a). Having a monadic result type means that source expressions do not have to be mapped directly to corresponding target expressions – they can also generate imperative code!
The ability to generate imperative code from expressions is crucial. Consider the Iter construct. Since there is no corresponding construct in LowExp, the only way we can translate this construct is by generating an imperative for-loop.
Translation of HighExp
Now that we have a general way to “re-rexpress” programs, we need to define the function that translates HighExp to LowExp. The first cases just map HighExp constructs to their corresponding LowExp constructs:
transHighExp :: HighExp a -> Prog LowExp (LowExp a)
transHighExp (HVar v) = return (LVar v)
transHighExp (HLit a) = return (LLit a)
transHighExp (HAdd a b) = LAdd <$> transHighExp a <*> transHighExp b
transHighExp (HMul a b) = LMul <$> transHighExp a <*> transHighExp b
transHighExp (HNot a) = LNot <$> transHighExp a
transHighExp (HEq a b) = LEq <$> transHighExp a <*> transHighExp b
It gets more interesting when we get to Let. There is no corresponding construct in LowExp, so we have to realize it using imperative constructs:
transHighExp (Let a body) = do
r <- initRef =<< transHighExp a
a' <- CMD $ GetRef r
transHighExp $ body $ valToExp a'
The shared value a must be passed to body by-value. This is achieved by initializing a reference with the value of a and immediately reading out that value. Note that we cannot use the smart constructor getRef here, because it would return an expression of type LowExp while body expects a HighExp argument. By using CMD $ GetRef r, we obtain a result of type Val, which can be translated to any expression type using valToExp.
Exercise: Change the translation of Let to use call-by-name semantics instead.
Translation of Iter is similar to that of Let in that it uses a reference for the local state. The iteration is realized by an imperative for loop. In each iteration, the state variable r is read, then the next state is computed and written back to the r. After the loop, the content of r is returned as the final state.
transHighExp (Iter n s body) = do
n' <- transHighExp n
r <- initRef =<< transHighExp s
for n' $ \_ -> do
sPrev <- CMD $ GetRef r
sNext <- transHighExp $ body $ valToExp sPrev
setRef r sNext
getRef r
Exercise: Change the translation of Iter by unrolling the body twice when the number of iterations is known to be a multiple of 2:
transHighExp (Iter (HMul n (HLit 2)) s body) = do
...
All that is left now is to put the pieces together and define the compile function:
translateHigh :: Prog HighExp a -> Prog LowExp a
translateHigh = reexpress transHighExp
compile :: Prog HighExp a -> String
compile = lowToCode . translateHigh
To see that it works as expected, we compile the powerInput example:
*EDSL_Compilation> putStrLn $ compile powerInput
printStr "Please enter two numbers\n"
printStr " > "
v0 <- readInput
printStr " > "
v1 <- readInput
printStr "Here's a fact: "
writeOutput v0
printStr "^"
writeOutput v1
printStr " = "
r2 <- initRef 1
for v3 < v1
v4 <- getRef r2
setRef r2 (v4 * v0)
end for
v5 <- getRef r2
writeOutput v5
printStr ".\n"
Note especially the for-loop resulting from the pure expression Iter n 1 (*m).
Discussion
The simplicity of the presented technique can be seen by the fact that most of the code is reusable.
Prog, interpret and reexpress are independent of the expression language.LowExp and its associated functions can be used as target for many different high-level languages.
The only (non-trivial) parts that are not reusable are HighExp and transHighExp. This shows that in fact very little work is needed to define an EDSL complete with code generation.
Generalized implementation
In fact, most of the code presented in this article is available in generic libraries:
Prog, interpret and reexpress are available in operational-alacarte (under the names Program, interpretWithMonad and reexpress).- Variants of the instructions in
CMD are available as separate composable types in the package imperative-edsl (which, in turn, depends on operational-alacarte).
As a proof of concept, we provide a supplementary file that implements the HighExp compiler using the generic libraries.
The whole compiler, not counting imports and the definition of HighExp, is just 30 lines of code! What is more, it emits real runnable C code instead of the pseudo-code used in this article.
Richer high-level languages
Part of the reason why transHighExp was so easy to define is that HighExp and LowExp support the same set of types. Things get more complicated if we want to extend HighExp with, for example, tuples.
Readers interested in how to implement more complex expressions using the presented technique are referred to the RAW-Feldspar source code (tag article-2016-03).
Appendix: Code generation
Here we define he missing parts of the code generator introduced in Code generation.
Code generation of instructions
-- Code generation monad
type Code = WriterT [Stmt] (State Unique)
type Stmt = String
type Unique = Integer
runCode :: Code a -> String
runCode = unlines . flip evalState 0 . execWriterT . indent
-- Emit a statement in the generated code
stmt :: Stmt -> Code ()
stmt s = tell [s]
-- Generate a unique identifier
unique :: String -> Code VarId
unique base = do
u <- get; put (u+1)
return (base ++ show u)
-- Generate a unique symbolic value
freshVar :: Type a => Code (Val a)
freshVar = ValComp <$> unique "v"
-- Generate a unique reference
freshRef :: Type a => Code (Ref a)
freshRef = RefComp <$> unique "r"
-- Modify a code generator by indenting the generated code
indent :: Code a -> Code a
indent = censor $ map (" " ++)
-- Code generation of instructions
codeCMD :: ShowExp exp => CMD exp a -> Code a
codeCMD (InitRef a) = do
r <- freshRef
stmt $ unwords [show r, "<- initRef", showExp a]
return r
codeCMD (GetRef r) = do
v <- freshVar
stmt $ unwords [show v, "<- getRef", show r]
return v
codeCMD (SetRef r a) = stmt $ unwords ["setRef", show r, showExp a]
codeCMD Read = do
v <- freshVar
stmt $ unwords [show v, "<- readInput"]
return v
codeCMD (Write a) = stmt $ unwords ["writeOutput", showExp a]
codeCMD (PrintStr s) = stmt $ unwords ["printStr", show s]
codeCMD (For n body) = do
i <- freshVar
stmt $ unwords ["for", show i, "<", showExp n]
indent $ code (body i)
stmt "end for"
Showing expressions
instance Show (Val a) where show (ValComp a) = a
instance Show (Ref a) where show (RefComp r) = r
class ShowExp exp where
showExp :: exp a -> String
bracket :: String -> String
bracket s = "(" ++ s ++ ")"
instance ShowExp LowExp where
showExp (LVar v) = v
showExp (LLit a) = show a
showExp (LAdd a b) = bracket (showExp a ++ " + " ++ showExp b)
showExp (LMul a b) = bracket (showExp a ++ " * " ++ showExp b)
showExp (LNot a) = bracket ("not " ++ showExp a)
showExp (LEq a b) = bracket (showExp a ++ " == " ++ showExp b)
Acknowledgement
This work is funded by the Swedish Foundation for Strategic Research, under grant RAWFP.
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