bindNonRec x r b produces either:

 let x = r in b

or:

 case r of x { _DEFAULT_ -> b }

depending on whether we have to use a case or let binding for the expression (see needsCaseBinding). It's used by the desugarer to avoid building bindings that give Core Lint a heart attack, although actually the simplifier deals with them perfectly well. See also MkCore.mkCoreLet

Given:

 case (C a b x y) of
        C b x y -> ...

We want to drop the leading type argument of the scrutinee leaving the arguments to match agains the pattern

This returns true for expressions that are certainly already evaluated to head normal form. This is used to decide whether it's ok to change:

 case x of _ -> e

into:

 e

and to decide whether it's safe to discard a seq. So, it does not treat variables as evaluated, unless they say they are. However, it does treat partial applications and constructor applications as values, even if their arguments are non-trivial, provided the argument type is lifted. For example, both of these are values:

 (:) (f x) (map f xs)
 map (...redex...)

Because seq on such things completes immediately.

For unlifted argument types, we have to be careful:

 C (f x :: Int#)

Suppose f x diverges; then C (f x) is not a value. However this can't happen: see CoreSyn. This invariant states that arguments of unboxed type must be ok-for-speculation (or trivial).

exprOkForSpeculation returns True of an expression that is:

• Safe to evaluate even if normal order eval might not evaluate the expression at all, or
• Safe not to evaluate even if normal order would do so

Precisely, it returns True iff:

• The expression guarantees to terminate,
• soon,
• without raising an exception,
• without causing a side effect (e.g. writing a mutable variable)

Note that if exprIsHNF e, then exprOkForSpecuation e. As an example of the considerations in this test, consider:

 let x = case y# +# 1# of { r# -> I# r# }
 in E

being translated to:

 case y# +# 1# of { r# -> 
    let x = I# r#
    in E 
 }

We can only do this if the y + 1 is ok for speculation: it has no side effects, and can't diverge or raise an exception.

Returns Just (dc, [x1..xn]) if the argument expression is a constructor application of the form dc x1 .. xn

The Arity returned is the number of value args the expression can be applied to without doing much work

Two expressions that hash to the same Int may be equal (but may not be) Two expressions that hash to the different Ints are definitely unequal.

The emphasis is on a crude, fast hash, rather than on high precision.

But unequal here means "not identical"; two alpha-equivalent expressions may hash to the different Ints.

We must be careful that \x.x and \y.y map to the same hash code, (at least if we want the above invariant to be true).

A cheap equality test which bales out fast! If it returns True the arguments are definitely equal, otherwise, they may or may not be equal.

See also exprIsBig