Tuesday, December 30, 2014

Higher order functions and interface files.

Work on DDC progresses, though recently it's been straightforward compiler engineering rather than anything novel, so I haven't been blogging. DDC now drops interface files when compiling modules, multi-module compilation works, and the front-end supports unicode string literals. I've also implemented a lambda lifter and support for higher order functions. This part is well baked enough to implement simple stream functions, as used in the Haskell Data.Vector library, though the compiler doesn't do the associated fusion yet. For example:
data Stream (s a : Data) where
        MkStream : (s -> Step s a) -> s -> Stream s a

data Step (s a : Data) where
        Yield   : a -> s -> Step s a
        Skip    : s -> Step s a 
        Done    : Step s a

-- | Apply a function to every element of a stream.
smap (f : a -> b) (ss : Stream s a) : Stream s b
 = case ss of  
    MkStream stepA sA0
        -> let stepB q = case stepA q of
                                Yield x sA1     -> Yield (f x) sA1
                                Skip sA2        -> Skip sA2
                                Done            -> Done
           in  MkStream stepB sA0

-- | Take the given number of elements from the front of a stream.
stake (n : Nat) (ss : Stream s a) : Stream (Tup2 s Nat) a
 = case ss of 
    MkStream fA sA0 
     -> let stepB q = case q of
                       T2 sA ix 
                        -> if ix >= n
                                then Done
                                else case fA sA of
                                        Yield x sA2 -> Yield x (T2 sA2 (ix + 1))
                                        Skip sA3    -> Skip  (T2 sA3 ix)
                                        Done        -> Done
        in   MkStream stepB (T2 sA0 0)
As we can see, work on larger demos is starting to be hampered by the lack of pattern matching sugar in the source language, so I'm going to add that next. After adding pattern matching, I'm going to spend a couple of weeks bug fixing, and should get the DDC 0.5 release out in early February 2015.

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