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Std.Word

packages/foundation/standard/src/Std/Word.alpha

1,783 lines192 declarations64.2 KiBSHA-256 27bf8c3f30ee
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def · lines 666–683

stdU16ToU8Checked

Full file
CHECKED NARROWING (L11n, NUM-005): `StdSome` exactly when the value is representable at the narrower width, `StdNone` otherwise (the wrapping narrows above keep the low bytes regardless). Unsigned: every dropped byte must be zero. Signed: every dropped byte must equal the sign byte of the kept part, so the value sign-extends back to itself.
666def stdU16ToU8Checked =
667  (lambda unrestricted value : (family StdU16) .
668    (eliminate
669      StdU16
670      (lambda unrestricted current : (family StdU16) . (family StdOption Byte))
671      value
672      (branch
673        StdU16Of
674        low
675        high
676        .
677        (nat-eliminate
678          (lambda unrestricted current : Nat . (family StdOption Byte))
679          (constructor StdOption StdNone Byte)
680          (lambda unrestricted predecessor : Nat .
681            (lambda unrestricted induction : (family StdOption Byte) .
682              (constructor StdOption StdSome Byte low)))
683          (byte-equal high (byte 0))))))

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