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95 lines
3.2 KiB
Idris
95 lines
3.2 KiB
Idris
module Data.String.Iterator
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import Control.Monad.Identity
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import public Data.List.Lazy
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%default total
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-- Backend-dependent string iteration type,
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-- parameterised by the string that it iterates over.
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--
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-- Beware: the index is checked only up to definitional equality.
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-- In theory, you could run `decEq` on two strings
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-- with the same content but allocated in different memory locations
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-- and use the obtained Refl to coerce iterators between them.
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--
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-- The strictly correct solution is to make the iterators independent
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-- from the exact memory location of the string given to `uncons`.
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-- (For example, byte offsets satisfy this requirement.)
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export
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data StringIterator : String -> Type where [external]
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-- This function is private
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-- to avoid subverting the linearity guarantees of withString.
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%foreign
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"scheme:blodwen-string-iterator-new"
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"C:stringIteratorNew"
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"javascript:stringIterator:new"
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private
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fromString : (str : String) -> StringIterator str
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-- This function uses a linear string iterator
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-- so that backends can use mutating iterators.
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export
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withString : (str : String) -> ((1 it : StringIterator str) -> a) -> a
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withString str f = f (fromString str)
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||| Runs the action `f` on the slice `res` of the original string `str` represented by the
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||| iterator `it`
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%foreign
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"scheme:blodwen-string-iterator-to-string"
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"C:stringIteratorToString"
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"javascript:stringIterator:toString"
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export
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withIteratorString : (str : String)
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-> (1 it : StringIterator str)
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-> (f : (res : String) -> a)
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-> a
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-- We use a custom data type instead of Maybe (Char, StringIterator)
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-- to remove one level of pointer indirection
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-- in every iteration of something that's likely to be a hot loop,
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-- and to avoid one allocation per character.
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--
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-- The Char field of Character is unrestricted for flexibility.
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public export
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data UnconsResult : String -> Type where
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EOF : UnconsResult str
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Character : (c : Char) -> (1 it : StringIterator str) -> UnconsResult str
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-- We pass the whole string to the uncons function
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-- to avoid yet another allocation per character
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-- because for many backends, StringIterator can be simply an integer
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-- (e.g. byte offset into an UTF-8 string).
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%foreign
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"scheme:blodwen-string-iterator-next"
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"C:stringIteratorNext"
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"javascript:stringIterator:next"
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export
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uncons : (str : String) -> (1 it : StringIterator str) -> UnconsResult str
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export
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foldl : (accTy -> Char -> accTy) -> accTy -> String -> accTy
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foldl op acc str = withString str (loop acc)
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where
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loop : accTy -> (1 it : StringIterator str) -> accTy
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loop acc it =
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case uncons str it of
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EOF => acc
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Character c it' => loop (acc `op` c) (assert_smaller it it')
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export
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unpack : String -> LazyList Char
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unpack str = runIdentity $ withString str unpack'
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where
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-- This is a Functor instance of Identity, but linear in second argument
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%inline
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mapId : forall a, b. (a -> b) -> (1 x : Identity a) -> Identity b
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mapId f (Id x) = Id (f x)
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unpack' : (1 it : StringIterator str) -> Identity (Lazy (LazyList Char))
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unpack' it =
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case uncons str it of
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EOF => pure []
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Character c it' => mapId (c ::) (unpack' $ assert_smaller it it')
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