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    <p><code>decodeFile</code> is not lazy, just look at the source -it calls <code>decodeOrFail</code>, which itself must parse the whole file to determine success or failure.</p> <p>EDIT:</p> <p>So what I believe worked in the original <code>binary</code> is now broken (read: it's now a non-lazy memory hog). One solution that I doubt is optimally pretty is to use lazy <code>readFile</code> and <code>runGetIncremental</code> then manually push chunks into the decoder:</p> <pre><code>import Data.Binary import Data.Binary.Get import Data.ByteString.Lazy as L import Data.ByteString as B import qualified Data.Array.IArray as IA import Data.Array.Unboxed as A main = do bs &lt;- getListLazy `fmap` L.readFile "bintest2.data" mapM_ doStuff bs return () doStuff b = print $ b IA.! 100000 </code></pre> <p>The important stuff is here:</p> <pre><code>getListLazy :: L.ByteString -&gt; [UArray Int Float] getListLazy lz = go decodeUArray (L.toChunks lz) where go :: Decoder (UArray Int Float) -&gt; [B.ByteString] -&gt; [UArray Int Float] go _ [] = [] go dec (b:bs) = case pushChunk dec b of Done b' o a -&gt; a : go decodeUArray (b' : bs) Partial f -&gt; case bs of (x:xs) -&gt; go (f $ Just x) xs [] -&gt; [] Fail _ _ s -&gt; error s -- alternatively use '[]' decodeUArray :: Decoder (UArray Int Float) decodeUArray = runGetIncremental get </code></pre> <p>Notice this solution didn't bother decoding then plumbing the list length through the decoder - I just changed up your generator code to output numerous arrays and not a list of arrays.</p> <p>To avoid code like this I think pipes would be the way to go.</p>
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    1. COThanks! This seems to be a rather harsh limitation, so I'm wondering if there's a easy way around it. For the time being, I'll try the following: Writing: Instead of encoding and writing a list of UArrays, I'll encode and write each UArray at a time. Reading: I'll read a chunk with defaultChunkSize, see if it decodes, otherwise read a bit more, and doStuff every time I have a complete chunk.
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    2. COI was thinking on this and was surprised to find `fmap decode (readFile f)` doesn't do it (it doesn't decode lists lazily). Now that I see this isn't trivially solved I'll give it another go in my free time.
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    3. CO@martingw Ok, I've posted an ugly but operational version that keeps the memory down. Notice that at this point you'd probably be well served by learning and using pipes (`readFileP >-> runStateP [] decode >-> doStuffP` or some such)
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