https://archetype-lang.org/assets/js/d640c751.3958cfaa.js

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1function m(e){let{components:t,...n}=e;return(0,r.kt)("wrapper",(0,a.Z)({},c,n,{components:t,mdxType:"MDXLayout"}),(0,r.kt)("p",null,"We present an improvement in the way ",(0,r.kt)("a",{parentName:"p",href:"/docs/asset"},"assets")," data are accessed in version ",(0,r.kt)("a",{parentName:"p",href:"/docs/installation"},(0,r.kt)("inlineCode",{parentName:"a"},"1.3.0")),"."),(0,r.kt)("h3",{id:"problems"},"Problems"),(0,r.kt)("p",null,"Until this version, the only way to access an asset data was the field accessor operator of the form ",(0,r.kt)("a",{parentName:"p",href:"/docs/reference/expressions/asset#ak--asset_keyaf"},(0,r.kt)("inlineCode",{parentName:"a"},"A[k].f")),", where ",(0,r.kt)("inlineCode",{parentName:"p"},"A")," is the asset collection, ",(0,r.kt)("inlineCode",{parentName:"p"},"k")," the asset key and ",(0,r.kt)("inlineCode",{parentName:"p"},"f")," the asset field."),(0,r.kt)("p",null,"The first problem was the access to multiple fields; for example, consider the following asset declaration:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-archetype"},"asset vehicle {\n  vin          : string;\n  manufacturer : string;\n  year         : nat;\n  nbdoors      : nat\n}\n")),(0,r.kt)("p",null,"Accessing several fields would end up accessing the underlying map several times:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-archetype"},'const k = "1G1AF1F57A7192174";\nconst m = vehicle[k].manufacturer;\nconst y = vehicle[k].year;\nconst n = vehicle[k].nbdoors\n')),(0,r.kt)("p",null,"Here the test and fail instructions are repeated three times."),(0,r.kt)("p",null,"The second problem is the ",(0,r.kt)("em",{parentName:"p"},"implicit")," fail of the ",(0,r.kt)("inlineCode",{parentName:"p"},"[]")," operator in situations where it is implicit that the asset exists."),(0,r.kt)("p",null,"It is typically the case when the asset field is accessed in the ",(0,r.kt)("inlineCode",{parentName:"p"},"true")," branch of a ",(0,r.kt)("a",{parentName:"p",href:"/docs/reference/expressions/asset#acontainsk--asset_keya"},(0,r.kt)("inlineCode",{parentName:"a"},"contains"))," test:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-archetype"},'const k = "1G1AF1F57A7192174";\nif vehicle.contains(k) then begin\n  const m = vehicle[k].manufacturer;\n  /* ... do something with m ... */\nend\n')),(0,r.kt)("p",null,"The ",(0,r.kt)("inlineCode",{parentName:"p"},"[].")," operator treats the case when asset is not found while it has already been tested."),(0,r.kt)("h3",{id:"solution"},"Solution"),(0,r.kt)("p",null,"Version ",(0,r.kt)("a",{parentName:"p",href:"/docs/installation"},(0,r.kt)("inlineCode",{parentName:"a"},"1.3.0"))," provides new operator ",(0,r.kt)("a",{parentName:"p",href:"/docs/reference/expressions/asset#ak--asset_keya"},(0,r.kt)("inlineCode",{parentName:"a"},"[]"))," that returns an ",(0,r.kt)("a",{parentName:"p",href:"/docs/reference/types#option%3CT%3E"},(0,r.kt)("inlineCode",{parentName:"a"},"option"))," of ",(0,r.kt)("a",{parentName:"p",href:"/docs/reference/types#asset_value%3CA%3E"},"asset value"),"."),(0,r.kt)("p",null,"Combined with the new ",(0,r.kt)("a",{parentName:"p",href:"/docs/reference/instructions/localvariable#-"},(0,r.kt)("inlineCode",{parentName:"a"},"?="))," declaration instruction, the proper way to retrieve all vehicle data presented above, is now as follows:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-archetype"},'const v ?= vehicle["1G1AF1F57A7192174"];\nconst m = v.manufacturer;\nconst y = v.year;\nconst n = v.nbdoors\n')),(0,r.kt)("p",null,"The declaration of ",(0,r.kt)("inlineCode",{parentName:"p"},"v")," fails with ",(0,r.kt)("inlineCode",{parentName:"p"},'"OPTION_IS_NONE"')," if vehicle is not found. It is possible to specify an error message with:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-archetype"},'const v ?= vehicle["1G1AF1F57A7192174"] : "VEHICLE_NOT_FOUND"\n')),(0,r.kt)("h3",{id:"single-field-access"},"Single field access"),(0,r.kt)("p",null,"Situations where a single asset field is accessed is very common though."),(0,r.kt)("p",null,"Version ",(0,r.kt)("a",{parentName:"p",href:"/docs/installation"},(0,r.kt)("inlineCode",{parentName:"a"},"1.3.0"))," provides a new operator ",(0,r.kt)("a",{parentName:"p",href:"/docs/reference/expressions/asset#ak--asset_keyaf"},(0,r.kt)("inlineCode",{parentName:"a"},"[]?."))," that returns an ",(0,r.kt)("a",{parentName:"p",href:"/docs/reference/types#option%3CT%3E"},(0,r.kt)("inlineCode",{parentName:"a"},"option"))," of field value, so that the non existence case may be ",(0,r.kt)("em",{parentName:"p"},"explicitely")," treated."),(0,r.kt)("p",null,"For example, the following retrieves an option of ",(0,r.kt)("a",{parentName:"p",href:"/docs/reference/types#nat"},(0,r.kt)("inlineCode",{parentName:"a"},"nat"))," to treat more specifically the case when the asset is not found:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-archetype"},'const opt_n = vehicle["1G1AF1F57A7192174"]?.nbdoors;

1\n/* treat case when on is none ... */\n')),(0,r.kt)("p",null,(0,r.kt)("inlineCode",{parentName:"p"},"opt_n")," is typed ",(0,r.kt)("inlineCode",{parentName:"p"},"option<nat>")," and is ",(0,r.kt)("inlineCode",{parentName:"p"},"none")," if the vehicle is not found."),(0,r.kt)("p",null,"Operator ",(0,r.kt)("a",{parentName:"p",href:"/docs/reference/expressions/asset#ak--asset_keyaf"},(0,r.kt)("inlineCode",{parentName:"a"},"[]."))," is still available: it fails when asset is not found, which is syntactically convenient when there is an ",(0,r.kt)("em",{parentName:"p"},"implicit")," existence invariant on the asset:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-archetype"},'const n = vehicle["1G1AF1F57A7192174"].nbdoors;\n')),(0,r.kt)("p",null,"It implictely fails with ",(0,r.kt)("inlineCode",{parentName:"p"},'("ASSET_NOT_FOUND", "vehicle")')," if ",(0,r.kt)("inlineCode",{parentName:"p"},'"1G1AF1F57A7192174"')," is not found."))}m.isMDXComponent=!0}}]);