diff options
Diffstat (limited to 'compiler/coreSyn/CoreSyn.lhs')
| -rw-r--r-- | compiler/coreSyn/CoreSyn.lhs | 49 |
1 files changed, 26 insertions, 23 deletions
diff --git a/compiler/coreSyn/CoreSyn.lhs b/compiler/coreSyn/CoreSyn.lhs index b36cb6d8a6..12a60daddd 100644 --- a/compiler/coreSyn/CoreSyn.lhs +++ b/compiler/coreSyn/CoreSyn.lhs @@ -180,25 +180,8 @@ These data types are the heart of the compiler -- /must/ be of lifted type (see "Type#type_classification" for -- the meaning of /lifted/ vs. /unlifted/). -- --- #let_app_invariant# --- The right hand side of of a non-recursive 'Let' --- _and_ the argument of an 'App', --- /may/ be of unlifted type, but only if the expression --- is ok-for-speculation. This means that the let can be floated --- around without difficulty. For example, this is OK: --- --- > y::Int# = x +# 1# --- --- But this is not, as it may affect termination if the --- expression is floated out: --- --- > y::Int# = fac 4# --- --- In this situation you should use @case@ rather than a @let@. The function --- 'CoreUtils.needsCaseBinding' can help you determine which to generate, or --- alternatively use 'MkCore.mkCoreLet' rather than this constructor directly, --- which will generate a @case@ if necessary --- +-- See Note [CoreSyn let/app invariant] +-- -- #type_let# -- We allow a /non-recursive/ let to bind a type variable, thus: -- @@ -359,9 +342,28 @@ See #letrec_invariant# Note [CoreSyn let/app invariant] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -See #let_app_invariant# +The let/app invariant + the right hand side of of a non-recursive 'Let', and + the argument of an 'App', + /may/ be of unlifted type, but only if + the expression is ok-for-speculation. + +This means that the let can be floated around +without difficulty. For example, this is OK: + + y::Int# = x +# 1# + +But this is not, as it may affect termination if the +expression is floated out: + + y::Int# = fac 4# + +In this situation you should use @case@ rather than a @let@. The function +'CoreUtils.needsCaseBinding' can help you determine which to generate, or +alternatively use 'MkCore.mkCoreLet' rather than this constructor directly, +which will generate a @case@ if necessary -This is intially enforced by DsUtils.mkCoreLet and mkCoreApp +Th let/app invariant is initially enforced by DsUtils.mkCoreLet and mkCoreApp Note [CoreSyn case invariants] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -1215,8 +1217,9 @@ mkDoubleLitDouble :: Double -> Expr b mkDoubleLit d = Lit (mkMachDouble d) mkDoubleLitDouble d = Lit (mkMachDouble (toRational d)) --- | Bind all supplied binding groups over an expression in a nested let expression. Prefer to --- use 'MkCore.mkCoreLets' if possible +-- | Bind all supplied binding groups over an expression in a nested let expression. Assumes +-- that the rhs satisfies the let/app invariant. Prefer to use 'MkCore.mkCoreLets' if +-- possible, which does guarantee the invariant mkLets :: [Bind b] -> Expr b -> Expr b -- | Bind all supplied binders over an expression in a nested lambda expression. Prefer to -- use 'MkCore.mkCoreLams' if possible |