purescript-transformerless
Why?
In Haskell and Purescript, the standard Writer, Reader, State and RWS
monads are implemented in terms of their monad transformer versions over the
Identity monad. Depending on how you learned about monad transformers, you might
remember reading something like the following:
The State/Reader/Writer monad is ...
A monad transformer is ...
The StateT/ReaderT/WriterT monad transformer is ...
In fact, the State/Reader/Writer monad from section {3 lines ago} is actually defined as StateT s Identity a/ReaderT r Identity/WriterT w Identity!
Wow, what a plot twist!
However, for all the theoretical cleanliness, it's "common knowledge" among Purescripters that transformer stacks are slow and generate some funky Javascript.
Usage
The same as a normal State, Reader, etc. monad. However, you should know that
none of these monads have instances for their respective transformer counterparts:
there is no instance for ..State.Class.MonadState s (..Transformerless.State s)
or its buddies. Wouldn't it be weird for a package called "transformerless" to
depend on a package called "transformers"?
As a result, a "transformers" typeclass function is just a normal function in the transformerless counterpart's module.
Questions I'll Ask and Answer for You
You mentioned code generation. Is it really better for this package?
Examples:
Generating code for this:
loop :: Int -> RWS String (Array String) Int Unit
loop n = tailRecM go n where
go 0 = do
RWS.tell ["Done!"]
pure (Right unit)
go n = do
x <- RWS.get
RWS.put (x + 1)
pure (Left (n - 1))with transformers:
var loop = function (n) {
var go = function (v) {
if (v === 0) {
return Control_Bind.bind(Control_Monad_RWS_Trans.bindRWST(Data_Identity.bindIdentity)(Data_Monoid.monoidArray))(Control_Monad_Writer_Class.tell(Control_Monad_RWS_Trans.monadWriterRWST(Data_Identity.monadIdentity)(Data_Monoid.monoidArray))([ "Done!" ]))(function () {
return Control_Applicative.pure(Control_Monad_RWS_Trans.applicativeRWST(Data_Identity.monadIdentity)(Data_Monoid.monoidArray))(new Data_Either.Right(Data_Unit.unit));
});
};
return Control_Bind.bind(Control_Monad_RWS_Trans.bindRWST(Data_Identity.bindIdentity)(Data_Monoid.monoidArray))(Control_Monad_State_Class.get(Control_Monad_RWS_Trans.monadStateRWST(Data_Identity.monadIdentity)(Data_Monoid.monoidArray)))(function (v1) {
return Control_Bind.bind(Control_Monad_RWS_Trans.bindRWST(Data_Identity.bindIdentity)(Data_Monoid.monoidArray))(Control_Monad_State_Class.put(Control_Monad_RWS_Trans.monadStateRWST(Data_Identity.monadIdentity)(Data_Monoid.monoidArray))(v1 + 1 | 0))(function () {
return Control_Applicative.pure(Control_Monad_RWS_Trans.applicativeRWST(Data_Identity.monadIdentity)(Data_Monoid.monoidArray))(new Data_Either.Left(v - 1));
});
});
};
return Control_Monad_Rec_Class.tailRecM(Control_Monad_RWS_Trans.monadRecRWST(Control_Monad_Rec_Class.monadRecIdentity)(Data_Monoid.monoidArray))(go)(n);
};without transformers:
var loop = function (n) {
var go = function (v) {
if (v === 0) {
return Control_Bind.bind(Control_Monad_Transformerless_RWS.bindRWS(Data_Semigroup.semigroupArray))(Control_Monad_Transformerless_RWS.tell([ "Done!" ]))(function () {
return Control_Applicative.pure(Control_Monad_Transformerless_RWS.applicativeRWS(Data_Monoid.monoidArray))(new Data_Either.Right(Data_Unit.unit));
});
};
return Control_Bind.bind(Control_Monad_Transformerless_RWS.bindRWS(Data_Semigroup.semigroupArray))(Control_Monad_Transformerless_RWS.get(Data_Monoid.monoidArray))(function (v1) {
return Control_Bind.bind(Control_Monad_Transformerless_RWS.bindRWS(Data_Semigroup.semigroupArray))(Control_Monad_Transformerless_RWS.put(Data_Monoid.monoidArray)(v1 + 1 | 0))(function () {
return Control_Applicative.pure(Control_Monad_Transformerless_RWS.applicativeRWS(Data_Monoid.monoidArray))(new Data_Either.Left(v - 1));
});
});
};
return Control_Monad_Rec_Class.tailRecM(Control_Monad_Transformerless_RWS.monadRecRWS(Data_Monoid.monoidArray))(go)(n);
};And what about speed?
On my computer, testing the above loop function (which is duplicated both on the
transformers repo and this one). The transformerless version is on the left, and
timing the loop function is labeled "RWS". The transformer version is labeled "RWST":
Installing
bower i purescript-transformerless
