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75 lines
2.5 KiB
Idris
75 lines
2.5 KiB
Idris
module Data.String.Iterator
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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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"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) -> ((it : StringIterator str) -> a) -> a
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withString str f = f (fromString str)
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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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"javascript:stringIterator:next"
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export
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uncons : (str : String) -> (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 -> (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 = withString str unpack'
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where
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unpack' : (it : StringIterator str) -> LazyList Char
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unpack' it =
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case uncons str it of
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EOF => []
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Character c it' => c :: Delay (unpack' $ assert_smaller it it')
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