#5083: Proposal: making inits and tails less strict
---------------------------------+------------------------------------------
Reporter: basvandijk | Owner:
Type: task | Status: new
Priority: normal | Component: libraries/base
Version: 7.0.3 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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I would like to propose making `inits` and `tails` less strict:
{{{
inits :: [a] -> [[a]]
-inits [] = [[]]
-inits (x:xs) = [[]] ++ map (x:) (inits xs)
+inits xs = [] : case xs of
+ [] -> []
+ x:xs -> map (x:) (inits xs)
}}}
{{{
tails :: [a] -> [[a]]
-tails [] = [[]]
-tails xxs@(_:xs) = xxs : tails xs
+tails xxs = xxs : case xxs of
+ [] -> []
+ _:xs -> tails xs
}}}
Having a lazier `inits` allows the elegant:
{{{nats = map length (inits nats)}}}
which loops for the current definition. This definition was [http://www
.mail-archive.com/[email protected]/msg21044.html due to John Tromp]
In the [http://thread.gmane.org/gmane.comp.lang.haskell.libraries/15459/
thread on the libraries list] there were some +1's and no objections.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5083>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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