| 64 | | (!ToDo: explain why all the above information is important, and what {{{DmdTypes}}} are.) |
| | 64 | (!ToDo: explain why all the above information is important) |
| | 65 | |
| | 66 | Though any expression can have a {{{Demand}}} associated with it, another datatype, {{{DmdType}}}, is associated with a function body. |
| | 67 | |
| | 68 | {{{ |
| | 69 | data DmdType = DmdType |
| | 70 | DmdEnv |
| | 71 | [Demand] |
| | 72 | DmdResult |
| | 73 | }}} |
| | 74 | A {{{DmdType}}} consists of a {{{DmdEnv}}} (which provides demands for all explicitly mentioned free variables in a functions body), a list of {{{Demand}}}s on the function's arguments, and a {{{DmdResult}}}, which indicates whether this function returns an explicitly constructed product: |
| | 75 | |
| | 76 | {{{ |
| | 77 | data DmdResult = TopRes -- Nothing known |
| | 78 | | RetCPR -- Returns a constructed product |
| | 79 | | BotRes -- Diverges or errors |
| | 80 | }}} |
| | 81 | |
| | 82 | The {{{dmdTransform}}} function takes a strictness environment, an [wiki:Commentary/Compiler/NameType Id] corresponding to a function, and a {{{Demand}}} representing demand on the function -- in a particular context -- and returns a {{{DmdType}}}, representing the function's demand type in this context. |
| | 83 | {{{ |
| | 84 | dmdTransform :: SigEnv |
| | 85 | -> Id |
| | 86 | -> Demand |
| | 87 | -> DmdType |
| | 88 | }}} |
| | 89 | Strictness analysis is implemented as a backwards analysis, so {{{dmdTransform}}} takes the demand on a function's result (which was inferred based on how the function's result is used) and uses that to compute the demand type of this particular occurrence of the function itself. |
| | 90 | |
| | 91 | {{{dmdTransform}}} has four cases, depending on whether the function being analyzed is a [wiki:Commentary/Compiler/EntityTypes data constructor] worker, an imported (global) function, a local {{{let}}}-bound function, or "anything else" (e.g., a local lambda-bound function). |
| | 92 | |
| | 93 | The data constructor case checks whether this particular constructor call is saturated. If not, it returns {{{topDmdType}}}, indicating that we know nothing about the demand type. If so, it returns a {{{DmdType}}} with an empty environment (since there are no free variables), a list of arg-demands based on the {{{Demand}}} that was passed in to {{{dmdTransform}}} (that is, the demand on the result of the data constructor call), and a {{{DmdResult}}} taken from the constructor Id's strictness signature. |
| | 94 | |
| | 95 | There are a couple of tricky things about the list of arg-demands: |
| | 96 | * If the result demand (i.e., the passed-in demand) has its box demanded, then we want to make sure the box is demanded in each of the demands for the args. (!ToDo: this may not be true) |
| | 97 | * If the result demand is not strict, we want to use ''n'' copies of {{{topDmd}}} as the list of arg-demands, where ''n'' is this data constructor's arity. |
| | 98 | |
| | 99 | (!ToDo: explain the other cases of {{{dmdTransform}}}) |
