Keean Schupke wrote:

Ahh, I see, perhaps you could give me a clue as to why (or how) this works... I have probably become a little over keen on using '$' as a general replacement for braces '(' ')' - the '$' was there because the actual wrapper function takes arguments and it looks neater not to have too many nested braces. I thought this could be something to do with runST enforcing strictness, but '$!' causes the same problem, only runST (wrapper x y z) appears to work - if its not too much trouble, how does runSTs type enforce this, and why does using '$' and '$!' cause type leakage?

Coincidentally, I tripped over this subtlety myself just last night. (I, too, often use '$' to avoid nested parentheses.) I concluded it was an instance of the "partial-application restriction" that I found described in section 7.11.4 of the GHC 5.02 User's Guide (still accessible at the GHC web site if you look carefully enough). [Simon: With a few minutes scanning, I couldn't find the same text in the current online User's Guide. Can you point out where it is?] -- Dean