Move pattern guards to CoreFn

[natefaubion]

Summary

Move pattern guards to CoreFn, and desugar them as part of codegen.

Motivation

backend-optimizer optimizes pattern-matching decision trees, but pattern guard desugaring commits too early to a particular control flow through it's CPS translation. If pattern guards were added to CoreFn, we would have more opportunity to analyze these cases and potentially add them to the existing matrix for additional sharing.

Proposal

Currently in CoreFn, a Guard for a case branch is an alias for Expr (ie, a single boolean expression).

purescript/src/Language/PureScript/CoreFn/Expr.hs

Lines 70 to 73 in 4afea2f

	-- |
	-- A guard is just a boolean-valued expression that appears alongside a set of binders
	--
	type Guard a = Expr a

This could be changed to something like:

-- |
-- A guard may bind an expression, or implicitly guard against a boolean expression
--
type Guard a = [(Maybe (Binder a), Expr a)]

-- |
-- An alternative in a case statement
--
data CaseAlternative a = CaseAlternative
  { -- |
    -- A collection of binders with which to match the inputs
    --
    caseAlternativeBinders :: [Binder a]
    -- |
    -- The result expression or a collect of guarded expressions
    --
  , caseAlternativeResult :: Either [(Guard a, Expr a)] (Expr a)
  } deriving (Eq, Ord, Show)

Alternatively, we could desugar all guards to a true <- ... pattern guard and ditch the Maybe.

[rhendric]

Currently, caseAlternativeResult is Either [(Guard a, Expr a)] (Expr a), where the left side covers boolExpr | rhs ; anotherBoolExpr | differentRhs .... How would that case be represented with this change?

[natefaubion]

Oops! You're right, I missed the outer [ ... ]. I've updated it to be NonEmpty.

Edit: Doh! That's not right either. I'll refactor this, but I think the idea still stands 😅

[natefaubion]

I've reverted the change to caseAlternativeResult. CoreFn is a bit off in general in that it allows empty lists where there shouldn't be, so I think just changing Guard is fine.