Source contract and expected records
This is an original, fully synthetic mapping example. Inputs are plain, acyclic JSON-shaped records with the fields shown; the canonical shape and rejection codes belong to this example. Adapt the contract before using a different source.
This custom syntax is field, one space, operator, one space, value. Declared types are integer or text; operators are =, >= and <=, but text accepts = only. Integer text permits an optional minus sign, no plus sign, and no leading zeros except 0; it must be in the safe range. For text, every character after the required separator belongs to the value, including leading, internal and trailing spaces. Return operator tags eq/gte/lte and retain source expressions.
| Case | Input record (JSON) | Expected output (JSON) |
|---|---|---|
| Parse typed constraint records | {"types":{"units":"integer","label":"text"},"expressions":["units >= 3","label = blue"]} | {"ok":true,"value":{"predicates":[{"field":"units","operator":"gte","value":3,"source":"units >= 3"},{"field":"label","operator":"eq","value":"blue","source":"label = blue"}]}} |
| Reject fraction for integer field | {"types":{"units":"integer"},"expressions":["units = 3.5"]} | {"ok":false,"error":"PREDICATE_INTEGER"} |
| Preserve text spacing | {"types":{"label":"text"},"expressions":["label = blue sky"]} | {"ok":true,"value":{"predicates":[{"field":"label","operator":"eq","value":"blue sky","source":"label = blue sky"}]}} |
Run the local transformation
Run the complete example as a JavaScript module in Node.js. It contains the pure mapping function, all three inputs and literal expected outputs; assertions stop on a mismatch. A rejected case returns ok=false with an error code, rather than a partial canonical record.
import assert from 'node:assert/strict';
function typedPredicates(i){
const fail=error=>({ok:false,error});if(!i||!i.types||typeof i.types!=='object'||Array.isArray(i.types)||!Array.isArray(i.expressions))return fail('PREDICATE_SHAPE');
const operators={'=':'eq','>=':'gte','<=':'lte'},predicates=[];
for(const source of i.expressions){
if(typeof source!=='string')return fail('PREDICATE_TEXT');const m=/^([a-z][a-z0-9_]*) (>=|<=|=) (.+)$/.exec(source);if(!m)return fail('PREDICATE_SYNTAX');
const [,field,op,raw]=m;if(!Object.hasOwn(i.types,field))return fail('PREDICATE_FIELD');let value;
if(i.types[field]==='integer'){
if(!/^-?(0|[1-9]\d*)$/.test(raw))return fail('PREDICATE_INTEGER');value=Number(raw);if(!Number.isSafeInteger(value))return fail('PREDICATE_INTEGER');if(Object.is(value,-0))value=0;
}else if(i.types[field]==='text'){if(op!=='=')return fail('PREDICATE_TEXT_OPERATOR');value=raw;}
else return fail('PREDICATE_TYPE');
predicates.push({field,operator:operators[op],value,source});
}
return {ok:true,value:{predicates}};
}
// Original synthetic cases; expected outputs are literal records.
const cases = [
{
"label": "Parse typed constraint records",
"input": {
"types": {
"units": "integer",
"label": "text"
},
"expressions": [
"units >= 3",
"label = blue"
]
},
"expected": {
"ok": true,
"value": {
"predicates": [
{
"field": "units",
"operator": "gte",
"value": 3,
"source": "units >= 3"
},
{
"field": "label",
"operator": "eq",
"value": "blue",
"source": "label = blue"
}
]
}
}
},
{
"label": "Reject fraction for integer field",
"input": {
"types": {
"units": "integer"
},
"expressions": [
"units = 3.5"
]
},
"expected": {
"ok": false,
"error": "PREDICATE_INTEGER"
}
},
{
"label": "Preserve text spacing",
"input": {
"types": {
"label": "text"
},
"expressions": [
"label = blue sky"
]
},
"expected": {
"ok": true,
"value": {
"predicates": [
{
"field": "label",
"operator": "eq",
"value": "blue sky",
"source": "label = blue sky"
}
]
}
}
}
];
for (const sample of cases) {
const actual = typedPredicates(sample.input);
assert.deepEqual(actual, sample.expected);
console.log(sample.label, JSON.stringify(actual));
}
Work through the boundary
Why is units = 3.5 rejected but label = blue sky retained?
Show the worked answer
units is declared integer, so a fraction returns PREDICATE_INTEGER. label is text: its full operand "blue sky" becomes a string value. Neither predicate is executed by this mapper.