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Brandon S Allbery KF8NH
Haskell is actually what manufacturing folks call "just in time"; things are evaluated when they are needed. Usually this means that when you output something, anything needed to compute that output will be done then. The exceptions are things like Control.Exception.evaluate (which you can treat as doing output but without *actually* outputting anything), mentioned above, plus you can indicate that some computation must be evaluated before another by means of Prelude.seq. You can also declare a type as being strict by prefixing an exclamation mark (so the runtime will always evaluate a computation before binding it), and with the BangPatterns extension you can also declare a pattern match binding as strict the same way.
Note that pattern matches are strict by default. In fact, a pattern match is the preferred way to force evaluation. Bang patterns only make sure that variables (i.e. wildcards, which wouldn't be evaluated otherwise) in a pattern match are evaluated to WHNF: case expr of Just x -> ... Nothing -> This is strict in the Maybe constructors, but non-strict in the argument of Just. When using a bang pattern, Just (!x) -> ... the match is also strict in the argument of Just. A bang pattern is really just a shortcut for using 'seq': Just x -> seq x $ ... or as some people prefer to write it: Just x | seq x True -> ... Greets, Ertugrul - -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) iQIcBAEBAwAGBQJNW6RSAAoJENVqN/rl3Y0RpmcQAI8no1y9NopXHKx0zCV+xV4W R2i2JdkPl8s7z+BpQLFqdKk+y9CfhqDLTwE+v8ek7fhnJXFWHqvGpi/1p6u3DRuO /ho8YNrzp8gKj0EF0Dn0VCwZ7qdmDBtmNXdHlMUzz2pTd968/9tK0FyUSb20qiBT Suv8lPv8shsGfYX29apo1JCcHRplfeS9wJpIk0/J5xaddTDElL0CBMWkehRFFlHi MVB3KS/6VnCqTCd8RtykzzmxtN2d+sf4a96h9RQWFt/62UPGfH04l2kY+1YiP6fK OaV0iqHyM1TLKT/tzp+duZ57TJ2MX5h00WczHduE01Y+7nnB8b67TAvUXJI21IwA loh1rHlqSoPI/lF1Ti5iJEF2K74waONtl7AM+lmOQZDVuipALQvesXSWTEvS18Mm fo052MUwzWgAXU4hwod5ZvjUNR92Z9vL5JAb1MP7DShE+sLJiXYDAD381XKU6FSi 0C4rMo553JXJcMNrnJhzdxDrfJCzcHIdePG6XOkH+EzRIUPs+mYlHuNTrFbZtyzC LzqfIwDMRQLo0f3KT5cj+6eDV/sUELW3seaTFUSCfIQhJ3molsimQHQy7YsJvvp7 fkcxpAfnAegiTQydfvUFFsdW+ZELeZTyW06iedMisUx1Lhww4Butee8PTZex3jHV qH/RghfnSK3w/cwiZez2 =ecix -----END PGP SIGNATURE-----