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Article contents:[[toc:]] [[tags: internals]] == Scrutinizer Scrutinizer is the CHICKEN's type checker. It checks type agreement in 2 situations: at function call sites and at assignments (set!). In addition to type checking, scrutinizer does optimizations by doing tree re-writes for function calls using specializations and special-cases ({{define-special-case}} in scrutinizer). === Type environment This is the big greek gamma letter one can see in the literature. It's basically just a mapping from variable identifiers to types. Things are added to and removed from it while the node tree is walked and variables enter and leave the current scope. It consists of three things: ; type database : This is populated from the .types files before the walking starts. ; e : The second argument to {{walk}}. id -> type alist ; blist : ("branch list"), when walking {{if}} nodes the variable types are updated only in the current {{if}} (tree) branch. New mappings are added to {{e}} when walking {{let}} nodes (only?). === Some functions ==== (match-types t1 t2 #!optional (typeenv (type-typeenv `(or ,t1 ,t2))) all) The heart of the scrutinizer. Let {{V_(t)}} denote the set of runtime values the type {{t}} stands for. Examples: {{V_(boolean) = {#t, #f}}} {{V_(integer) = {0, 1, -1, 2, -2, ...}}} {{V_((or boolean integer)) = {#t, #f, 0, 1, -1, 2, -2, ...}}} When {{all}} is true {{match-types}} returns {{#t}} iff {{V_(t2)}} is a subset of {{V_(t1)}}. When {{all}} is false {{match-types}} returns {{#t}} iff the sets {{V_(t2)}} and {{V_(t1)}} intersect. If {{all}} is true and {{match-types}} is true then it's safe to specialize. If {{all}} is false and {{match-types}} if false it's safe to issue warning without risking a false positive. In addition, this function mutates {{typeenv}} and the {{trail}} if unification happens. ==== scrutinize -> (walk n e loc dest tail flow ctags) ; {{n}} : the canonicalized node (tree) ; {{e}} : one part of "type environment". An alist from identifiers to type types. ; {{flow}} and {{ctags}} : related to blist Returns {{*}} for unknown number of return values, or {{(type ...)}} for known number of return values. For example walking {{(begin)}} would return {{*}}. Walking {{(cons 'a 'b)}} would return {{((pair symbol symbol))}}. ==== (refine-types t1 t2) Given a variable has type {{t1}} and it's learned that it also has type {{t2}} this function returns a hopefully more detailed type for the variable. There are two cases when this function is used: Refining variable's type for if's branches when the if's test was a predicate: <enscript highlight="scheme"> (let ((x (the (list-of symbol)))) (if (pair? x) <branch-c> <branch-a>)) </enscript> Here {{pair?}} is a predicate for {{pair}}. In {{<branch-c>}} we know {{x}}'s type is {{pair}} in addition to {{'(list-of symbol)}}. The variable's type will be updated, in {{blist}}, to {{(refine-types '(list-of symbol) 'pair)}} which would ideally return {{(pair symbol (list-of symbol))}}. For {{<branch-a>}} {{x}}'s type will be {{(refine-types '(list-of symbol) '(not pair))}} which would ideally return {{null}}. This kind of reasoning is called {{occurence typing}} in the literature. In {{tests/scrutinizer-tests.scm}} the last case could be asserted with this: {{(test (~> (list-of symbol) (not pair) null))}} Second place refine-type will be used is when calling functions with {{enforce-argument-types}} declaration: <enscript highlight="scheme"> (let ((x (foo))) (length x)) </enscript> This will cause a {{x}}'s type to be set (in blist) to {{(refine-types '* 'list)}}, which should be {{list}}. {{list}} is an alias for {{(list-of *)}}. * Ideally if {{t2}} is a subtype of {{t1}} (t2 <= t1 and {{(match-types t1 t2)}} returns true) refine-types should return {{t2}}. Simple case: {{(refine-types '(or symbol number) 'symbol) -> 'symbol}}. === TODO * how function calls are checked * enabling debug output === possible topics * what is type * subtype relation * typeenv / forall / type variables / unification * how match-types works Description of your changes:

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