Stream package:ghc

Monadic streams

Stream m a b is a computation in some Monad m that delivers a sequence of elements of type a followed by a result of type b. More concretely, a value of type Stream m a b can be run using runStreamInternal in the Monad m, and it delivers either

• the final result: Done b, or
• Yield a str where a is the next element in the stream, and str is the rest of the stream
• Effect mstr where mstr is some action running in m which generates the rest of the stream.

Stream is itself a Monad, and provides an operation yield that produces a new element of the stream. This makes it convenient to turn existing monadic computations into streams. The idea is that Stream is useful for making a monadic computation that produces values from time to time. This can be used for knitting together two complex monadic operations, so that the producer does not have to produce all its values before the consumer starts consuming them. We make the producer into a Stream, and the consumer pulls on the stream each time it wants a new value. Stream is implemented in the "yoneda" style for efficiency. By representing a stream in this manner fmap and >>= operations are accumulated in the function parameters before being applied once when the stream is destroyed. In the old implementation each usage of mapM and >>= would traverse the entire stream in order to apply the substitution at the leaves. The >>= operation for Stream was a hot-spot in the ticky profile for the ManyConstructors test which called the cg function many times in StgToCmm.hs

Give even more information on the source than getTokenStream This function allows reconstructing the source completely with showRichTokenStream.

Return module source as token stream, including comments. A Module can be turned into a ModSummary using getModSummary if your session is fully initialised. Throws a SourceError on parse error.

Take a rich token stream such as produced from getRichTokenStream and return source code almost identical to the original code (except for insignificant whitespace.)

The Stream instantiation used for code generation. Note the underlying monad is UniqDSMT IO, where UniqDSMT is a transformer that propagates a deterministic unique supply (essentially an incrementing counter) from which new uniques are deterministically created during the code generation stages following StgToCmm. See Note [Object determinism].