Question

Consider the following:

do putStr ""
   return $ map read ["2"]

Run in GHCI, this gives

[*** Exception: Prelude.read: no parse

I expected an “ambigious type variable” compilation error, but instead this comes up at runtime. Why?

For comparison, the following three examples do give “ambiguous type variable” errors:

map read ["2"]

do putStr ""
   return $ read "2"

do [1]
   return $ map read ["2"]

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By the way, here’s the original code where I encountered this.

readInts :: String -> IO [Integer]
readInts f = do s <- readFile f
                return $ map read $ splitOneOf " \n" s

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This has been resolved. My original code attempted to read from “” at the end of the list. I received the same exception for a different reason when I investigated in GHCI.

Answer 1

This happens because of GHCi’s extended default rules. If you put the same code in a file without a signature, you should see the error you expected.

(In this case it’s probably picking () as the type, which is why you get a parse error.)