[Haskell-cafe] Learn You a Haskell for Great Good - a few doubts
wren ng thornton
wren at freegeek.org
Tue Mar 8 02:11:07 CET 2011
On 3/7/11 6:58 PM, Alexander Solla wrote:
> The "magic" semantics of evaluating the first argument are done by the
> compiler/runtime, and are apparently not expressible in Haskell.
Of course this is true. The only ways of forcing evaluation in Haskell
are (a) to perform pattern matches on a value, (b) use 'seq'--- either
directly or in its disguised forms: strict data constructors and
In order to use pattern matching you need to know what the constructors
of the type are in order to force a choice between different
constructors; therefore you can't use case analysis to define a function
with the type and semantics of 'seq'.
But 'seq' is still defined in the Haskell report and therefore a part of
Haskell. Many have lamented the problems introduced by a parametric
polymorphic 'seq'; if it were just type-class polymorphic then it
wouldn't be such a problem. But then a type-class polymorphic 'seq'
could lead to maintenance issues similar to those faced by Java's
checked exceptions, which is why it was rejected from Haskell.
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