Rational -> Float
Raise any value as an exception, provided it is in the Typeable class.
Type extension of builders for type constructors
Application operator. This operator is redundant, since ordinary application (f x) means the same as (f $ x). However, $ has low, right-associative binding precedence, so it sometimes allows parentheses to be omitted; for example:
> f $ g $ h x = f (g (h x))
It is also useful in higher-order situations, such as map ($ 0) xs, or Data.List.zipWith ($) fs xs.
Strict (call-by-value) application, defined in terms of seq.
Evaluates its first argument to head normal form, and then returns its second argument as the result.
Indicates that it may be beneficial to evaluate the first argument in parallel with the second. Returns the value of the second argument.
a `par` b is exactly equivalent semantically to b.
par is generally used when the value of a is likely to be required later, but not immediately. Also it is a good idea to ensure that a is not a trivial computation, otherwise the cost of spawning it in parallel overshadows the benefits obtained by running it in parallel.
Note that actual parallelism is only supported by certain implementations (GHC with the -threaded option, and GPH, for now). On other implementations, par a b = b.
Semantically identical to seq, but with a subtle operational difference: seq is strict in both its arguments, so the compiler may, for example, rearrange a `seq` b into b `seq` a `seq` b. This is normally no problem when using seq to express strictness, but it can be a problem when annotating code for parallelism, because we need more control over the order of evaluation; we may want to evaluate a before b, because we know that b has already been sparked in parallel with par.
This is why we have pseq. In contrast to seq, pseq is only strict in its first argument (as far as the compiler is concerned), which restricts the transformations that the compiler can do, and ensures that the user can retain control of the evaluation order.
Extend a generic builder
Build a term and use a generic function for subterms
Runs a Reader and extracts the final value from it. (The inverse of reader.)
Evaluate a state computation with the given initial state and return the final value, discarding the final state.
* m s = fst (runState m
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